题目

“Tokyo seeking international support over the Diaoyu Islands will be ______,” Chinese Foreign Ministry spokesman Lu Kang said on Tuesday. A . in vain B . in silence C . in ruins D . in chorus 答案：A 一颗彗星沿一条以地球为焦点的抛物线轨道运行(设彗星、地球都是质点)时，当彗星离地球d万公里时，经过地球和彗星的直线与抛物线对称轴的夹角为30°，求此彗星运行时离地球最短的距离．