Only after he had spent several nights working on the math problem _______ to wonder whether the problem was wrongly set.

A．that he began

B．did he begin

C．then he began

D．so that he began

把铁片分别放入下列物质的溶液中，过一段时间取出，溶液质量增加的是[ ]

A.Al₂(SO₄)₃

B.CuSO₄

C.H₂SO₄

D.AgNO₃