Question

——　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

Answer 1

B

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。