问题

_________life began on the earth is one of thebiggest puzzles to scientists

A:should be
B:should have been
C:must be
D:might have been

正确答案是：

同类题型

“When my husband died five years ago，__________ was difficult to go from ‘we’ to ‘me’，”said Joe Kita to us.

Paulseems upset today．—Yes，he wrote an application letter toNeusoft but waited__________for a reply .A．in vain B．inorder C．in time D．inreturn

Who were potato chips invented by

May I remind you that MrWhite is waiting outside, MrsGreen? —Oh, I _________ all about it

Only when ______ hard ______ make your dream come true.