预应力结构构件的混凝土,需满足下列要求,何者是正确的()。
A.强度高
B.收缩.徐变小
C.快硬.早强
D.强度等级不宜低于C20
参考答案:A, B, C
Let R and S be the relation respectively: Relation R: A B C Relation S: B C D 1 2 3 2 3 4 6 7 8 2 3 5 The number of tuplesthat the result of the expression R ∞ S (Here is the natural join of relations R and S) is
A.3
B.4
C.6
D.9
容积残余变形率一般情况下不得大于()
A.5%
B.8%
C.10%