Friends are angels who lift us to our feet _____ our wings have trouble remembering how to fly. [ ]

A. unless          

B. because       

C. though            

D. when

设有关系R和S．关系代数表达式R-（R-S）表示的是（）

A.R∩S 

B.R-S 

C.R∪S 

D.R÷S