OG 13 SC 13题

michellekongfu

There are several ways to build solid walls using just
mud or clay, but the most extensively used method
has been the forming of bricks out of mud or clav.
and, after some preliminary air drying or sun drying.
thev are laid in the wall in mud mortar.

(A) the forming of bricks out of mud or clay, and,
after some preliminaryair drying or sun drying,
they are laid
(B) forming the mud or clay into bricks, and, after
some preliminary air drying or sun drying, to
lay them
(C) having bricks formed from mud or clay, and,
after some preliminaryair drying or sun drying,
they were laid
(D) to form the mud or clay into bricks, and, after
some preliminaryair drying or sun drying, to
lay them
(E) that bricks were formed from mud or clay,
which, after some preliminary air drying or sun
drying, were laid

虽然可以用平行选出正确答案，但是D中的them为什么可以指代bricks?