题目

完型填空     阅读短文，从各小题所给的A、B、C、D四个选项中选出最佳答案。 What is the sky? Where is it? How high is it? What lives above it? These questions are difficult   1  ， 2  they? Is the sky blue? The sky   3  no colour. We know that there is air around   4   world. When planes fly, they need air   5  their wings. Planes can not fly very high because when they go higher, the air gets   6  ．If we go up about 1,200 kilometers  7   the earth, we find there is   8  air of all. Maybe we can answer   9   of our questions no设Sn是正项数列{an}的前n项和，4Sn=a2n+2an-3．（I）求数列{an}的通项公式；（II）已知bn=2n，求Tn=a1b1+a2b2+…+anbn的值．