混凝土的收缩
参考答案：
不受荷载作用的混凝土在空气中结硬时，其体积会变小的现象称为收缩。
职业病防治法共有（）章，（）条。
A、7，78
B、7，79
C、8，80
He ___________ the volunteer group last year and ___________ many activities since then.
A. took part in…joined B. joined…took part in C. joined…has taken part in
