名词性从句：The boy dived into the water and after---seemed to be

名词性从句：The boy dived into the water and after___seemed to be a long time ,he came up again.
The boy dived into the water and after___seemed to be a long time ,he came up again.
A which
C what
D it
1.请问为何不选D,而要选C?请您详细解讲好吗?
2.这是宾语从句吗?
3.which 在名词性从句中除了做定语,可以做主语和宾语吗?（因为其在定从中就做主宾）想得到明确的答案,书上总是说得很笼统. 谢谢啦
一楼的朋友，你没有回答我which在名词性从句中做什么成分