it took him ten years to complete it.that's how long----to d

选1,it taker(took) Sb 。。。to do sth,意为做某事（to do sth）话费了某人多少时间,你所出题目的第一句刚好就是一个例子。所要填空的句子是个表语从句,意思是“那是花了他多少时间来完成它”本来应该是“he took how long to do it"因为做了表语从句,所以将how long提前,充当领导词,所以选1。