— You don't forget when we borrowed that reference book,  do you?  

— I can't remember now but  ___ sometime last week?     [ ]

A. might it be                  

B. could it be  

C. could it have been          

D. should it have been

二次应力Q是指（）

A.由相邻部件的约束或结构的自身约束所引起的压应力或切应力

B.由相邻部件的约束或结构的自身约束所引起的拉应力或切应力

C.由相邻部件的约束或结构的自身约束所引起的正应力或切应力