若数列的前n项的和,那么这个数列的通项公式为 A. B. C. D. 答案:D用适当的介词填空。1. Please put _________ your winter jacket. It's cold. 2. I came _________ Friday. 3. Good morning, class! Look _________ the window! 4. They are getting ready _________ school. 5. I like to ski _________ the snow.