As we approached the work site,the workers were seen_

问题描述:

As we approached the work site,the workers were seen_
As we approached the work site,the workers were seen_the newv house.
A.building
B.build
C.built
D.to build
选了D,想问问为什么选A不选D

当我们接近建筑工地时,发现工人们正在干活
ing形式,表示当时看见某人正在做某事