（）是指砖、灰土、三合土、毛石、混凝土、毛石混凝土等材料组成的墙下条形或柱下独立基础。
参考答案：刚性基础
—What a nice classroom!
—It _____ every day. [ ]
A. is cleaning
B. has cleaned
C. must clean
D. is cleaned
