Answer: Option C. -> radiation
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C
Radiation is the mode of heat transfer between the earth and the sun.Radiationis amethod of heat transferthat does not rely upon any contact between the source and the object. The transfer of heat by radiation does not require any medium. It can take place whether a medium is present or not.

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Modes: 1 Mark each
The various ways in which heat is transferred are:
(i) Conduction: In this mode, heat is transferred between adjacent molecules through vibration, without the actual movement of the molecules. This type of heat transfer happens in metals.
(ii) Convection: In air and water, heat is transferred by convection. The hot air becomes lighter and moves upward which is then replaced with the colder particles from the top and sides. In convection, the actual movement of molecules takes place.
(iii) Radiation: This mode of heat transfer doesn't require any medium for its transfer. All hot bodies radiate heat. When heat falls on an object, a part of it is absorbed, a part of it is reflected and a part of it is transmitted. The temperature of the body rises due to the absorbed part of the heat.

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(a) Reason: 1 Mark
(b) Working: 2 Marks
(a) Hot air rises up because, it has lesser density than the cooler air around it.
(b) A hot-air balloon works on the principle that hot air rises up.
It consists of a burner and a balloon envelope. When the burner is lit, it heats the air around it.
The hot air rises up and the cold air on the sides comes near the burner. The same process continues for a long time.
The hot air gets trapped in the balloon and the hot air balloon rises, depending on the amount of hot air in it..

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(a) Land and Sea breeze: 2 Marks
Radiation: 2 Marks
(b) Explanation: 1 Mark
(a) Water takes a bit more time to warm up and is able to retain the heat longer than land does.In the day, when the sun is up, the land heats up very quickly and the air above it warms up a lot more than the air over the water. The warm air over the land is less dense and begins to rise. Low pressure is created.The air pressure over the water is higher with cold dense air, which moves to occupy the space created over the land. The cool air that comes along is called a sea breeze.
In the night, the reverse happens. The land quickly loses itsheat whiles the water retains itswarmth. This means the air over the water is warmer, air is less dense and begins to rise. Low pressure is created over the water. Cold and dense air over the land begins to move to the water surface to replace the warmer rising air. The cool breeze from the land is called a land breeze.
Convection is the process responsible for land and sea breeze. Convection is the process of transfer of heat from one place to another by the actual movement of the particles. Particles carry heat from one place to another.
Radiation is the process in which heat flows from one object to another object either through the medium or throughvacuum. In convection, a medium is necessary for heat transfer from one end to another but in radiation, no medium is required for transferring of heat.
(b) Sun warms the earth by the process of radiation.

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Definition: 0.5 Marks
Example: 0.5 Marks
Temperature is the degree of the hotness or coldness of a body.
Degree Celcius ( ${}^{\circ }C$), degree Fahrenheit ( ${}^{\circ }F$) and Kelvin(K)are the units of temperature.

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Difference: 2 Marks
Cooking: 1 Mark
Conduction: Conduction is the process of transfer of heat from the hotter part to the colder part of an object without the movement of the particles. Also, in conduction, heat gets transferred between substances that are in direct contact with each other.
Convection: It is the process of transfer of heat by the movement of particles of a medium from one place to another. It takes place in liquid and gases.
Cooking of food usually uses a combination of conduction, convection and radiation. Conduction transfers the heat using direct contact; food is heated directly in a metal pan, in a liquid, or surrounded by air. Dropping an egg into a pan of boiling water is a good example.Convection occurs by the movement of air, liquid, or steam around the food. For example, as a pan of soup heats up on the stove, heat moves from the bottom of the pan. Heat transfer by radiation occurs when microwave (light waves) or infrared energy (heat waves) isspread into the food. As the microwaves penetrate the food, they bump into molecules of water and fat, causing them to vibrate rapidly. This vibration creates friction, which creates heat that cooks the food.

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Range: 1 Mark
Precautions: 2 Marks
Therange of a laboratory thermometer is -10˚C to110˚C.
We need to take the following precautions while taking the readings from a laboratorythermometer:
i) It should be kept upright and not tilted while taking the observation.
ii) The bulb should be surrounded on all the sides by the substance of which the temperature is to be measured.
iii)The bulb should not touch the container.

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Each part: 1 Mark
(i) The normal body temperature of a human is 37${}^{\circ }$C.
(ii) It cannot reach us by conduction or convection as there is no mediumin space between the earth and the sun. From the sun the heat comes to us byradiation.

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Definition: 1 Mark
Range: 1 Mark
A clinical thermometer is a temperature measuring deviceused to measure only the body temperature.
The range of a clinical thermometer is 35${}^{\circ }$C to 42${}^{\circ }$C.

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Each option: 1 Mark
(a) The general mode of heat transfer in solids is conduction.
(b) The general mode of heat transfer in liquids and gases is convection.