题目

 It was in 1969 ______ two Americans went to the moon by spaceship. A. when             B. that           C. at that time               D. just then 答案：B某元素X的最高价氧化物的分子式为X2O5，在它的气态氢化物中含氢3.85%，则该元素的相对原子质量为(    ) A．14               B．31                 C．74.9                D．121.8