OG12 11，为什么用完成时

henguc

There are several ways to build solid wallsusing just mud or clay, but the most extensively used method has been theforming of bricks out of mud or clay, and, after some preliminary air drying orsun drying, they are laid in the wall in mud mortar.
A.    theforming of bricks out of mud or clay, and, after some preliminary air drying orsun drying, they are laid
B.     formingthe mud or clay into bricks, and, after some preliminary air drying or sundrying, to lay them
C.    havingbricks formed from mud or clay, and, after some preliminary air drying or sundrying, they were laid
D.    toform the mud or clay into bricks, and, after some preliminary air drying or sundrying, to lay them     (D)
E.     thatbricks were formed from mud or clay, which, after some preliminary air dryingor sun drying, were laid
题目我会做，是典型的平行，但是他的完成时用得让我觉得很别扭。是表示这种方法延续到现在吗？