求问have/has been to的用法

不想再混吃等死

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 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



如这一题， 我理解D选项parallelism原则，但是不明白has been后加to form的意思， 为什么has been后可以加to do呢？
求大家帮忙解释，谢谢！

周小七cc

在OG的解释里已经说了，"to form"是是平行前面的“to build”，后面这句省略了way，完整的理解应该是：the most extensively used method has been (the way) to form the...