题目

–Do you know what speed _______ for a rocket to escape from the pull or gravity of the earth to send a satellite into outer space?    ---No idea at all. I have never got the ______ of it.     A. it needs; idea      B. is taken; hang      C. is needed; hang     D. it takes; thought   答案：C分子、原子、离子都是构成物质的微粒．下列物质由离子构成的是（ ）A.二氧化碳B.黄金C.水D.氯化钠