This question asks you to identify an assumption underlying the last sentence of the passage. In this sentence, the author of the passage draws a conclusion about the age of the Geminid stream. This conclusion is based on two pieces of information. The first is the length of time the Earth would take to cross the computer-model Geminid stream if the stream were 5,000 years old. The second is the actual elapsed time between the two peaks of meteor activity predicted by the computer model. In concluding from this information that the Geminid stream is actually only 3,000 years old, the author is assuming the accuracy of the computer model, as stated in D, the best answer. Choice A is incorrect because the passage says that the time the Earth takes to cross the stream would vary from year to year and that 19 hours was the average time, not the exact time, observed from 1970 to 1979. Choices B and C are incorrect because the passage does not suggest anything about the current state of the comet associated with the Geminid stream or about the expected longevity of the stream.