It was playing computer games_______ cost the boy plenty of time______ he ought to have spent doing his homework.    

A．that; that

B．which; that

C．which; when

D．that; when

用硝酸银滴定法测定水中氯化物，配制氢氧化铝悬浮液时用倾写法反复洗涤沉淀物，这是为了去除氯离子产生的正误差。