—— Was that the new er who walked by?
—— ＿＿＿＿＿.
A. It must be that
B. It must have been
C. He must be
D. This must have been
网友回答
A. It must be that
(答案→)B. It must have been
C. He must be
D. This must have been
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 er。