请教

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.

1. the forming of bricks out of mud or clay, and, after some preliminary air drying or sun drying, they are laid

2. forming the mud or clay into bricks, and, after some preliminary air drying or sun drying, to lay them

3. having bricks formed from mud or clay, and after some preliminary air drying or sun drying , they were laid

4. to form the mud or clay into bricks, and , after some preliminary air drying or sun drying, to lay them

5. that bricks were formed from mud or clay, which, after some preliminary airy drying or sun drying, were laid

 答案说:

D the best

The method to…       and前后平行

可是不明白 has been to form 这个结构, has been 这里是什么意思, 做什么成分?