Sentence Correction

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 clay.
and, after some preliminary air drying or sun drying.
they are laid in the wall in mud mortar.

(A) the forming of bricks out of mud or clay, and,
after some preliminary air 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 preliminary air drying or sun drying,
they were laid
(D) to form the mud or clay into bricks, and, after
some preliminary air 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

OA： D

my confusion: why form and lay should be parallel with to build? I understand that the first two must be parallel with each other, but why they should be with"to build" ?