He didn't know that_____ten years to complete this special task.A. it will take himB. he would takeC. he would spendD. it would take him

步骤 1：分析句子结构
句子的主句是"He didn't know that"，后面跟的是一个宾语从句。宾语从句的主语是"ten years"，宾语从句的谓语动词需要表达"花费"的意思。

步骤 2：确定宾语从句的时态
由于主句使用了过去时态"He didn't know"，所以宾语从句也应使用相应的过去时态，即"would take"。

步骤 3：确定宾语从句的主语和宾语
宾语从句的主语是"ten years"，宾语是"this special task"。在英语中，表示"花费时间做某事"的结构是"it takes sb. some time to do sth."，因此宾语从句的主语应为"it"，宾语从句的宾语是"him"。