题目

  —— Was that the new comer who walked by?—— ＿＿＿＿＿.A.It must be that     B.It must have been  C.He must be     D.This must have been 答案：B 解析: 表示对过去情况的猜测，用“情态动词＋have done”；而it代替上文所指的new comer。下列有关叙述正确的是(    )A.在CCl4和CH4中各原子最外层均达到8个电子稳定结构B.在MgO晶体中，O2-的结构示意图可表示为C.13C与14C互为同素异形体D.NH3是非极性分子，而CO2是极性分子