英语单项选择 He didn't know that-------ten years to complete this

英语单项选择 He didn't know that-------ten years to complete this special task.
He didn't know that-------ten years to complete this special task
A:it will take him
B:he would take
C:he would spend
D:it would take him
请问为什么选D.难道一定需要IT做形式宾语吗?
希望能解释为什么不选其他3个.这是一类问题我都不太懂,希望有详细解释这题
谢谢