You_____ all those calculations! We have a computer to do that sort of thing.       [      ]

A. needn't have done             

B. must not have done       

C. shouldn't have done            

D. can not have done

下列有关股票与债券的区别，认识正确的是（）

A.发行股票是为了敌措资金，发行债券是为了平衡财政收支

B.股票具有风险性，债券则没有风险性

C.股票买卖有时间限制，债券可以随时还本付息

D.股票以取得股息和红利为补偿条件，债券以定期收取利息为条件