In each question, the first and the last sentences of the passage are numbered 1 and 6 respectively. The rest of the passage is split into four parts. These four sentences are jumbled. Read the sentences and identify their correct and logical order. According to an engineerP : might hit the market next yearQ : a newly developed air-cooler systemR : that employed in conventional room coolersS : which is based on a principle radically different fromThe proper sequence should be: