A、B、C、D、E是同一周期的五种主族元素,A和B的最高价氧化物对应的水化物均呈碱性,且碱性B>A,C和D的气态氢化物的稳定性C>D;E是这五种元素中简单离子半径最小的元素,则它们的原子序数由大到小的顺序是 A.A、B、C、D、E B.C、D、E、A、B C.B、A、D、C、E D.C、D、A、B、E 答案:C
. Tom as well as his classmates __________
a song when the teacher came into the classroom.
A.are singing
B.were singing
C.is singing
D.was singing