题目选项分析

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

There are several ways to build solid walls using just mud or clay, but the most extensively used method has been to form the mud or clay into bricks, and, after some preliminary air drying or sun drying, to lay them in the wall in mud mortar.