and前加逗号的疑问

理上网来

OG12－SC11 and前面逗号的疑问
There are several ways to build solid walls using just mud or clay, but the most extensivelyusedmethod 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


Key：D

我不太明白这个and前面为什么要加个逗号，我觉得如果在and前加逗号，那么and后就应该接一个完整的句子而不是一个短语。所以很快排除了D选项。after……的插入语应该是可以删除而不影响句子结构的。求NN们解疑。