有抗冻要求的混凝土且环境水兼硫酸盐侵蚀时,应优先选用的水泥品种是( )。
A.普通硅酸盐水泥
B.硅酸盐大坝水泥
C.抗硫酸盐硅酸盐水泥
D.硅酸盐水泥
参考答案:C
I _________ him before. But my friends _________ him two days ago. [ ]
A. didn't meet; met
B. haven't met; met
C. don't meet; meet
D. hadn't met; meet
下列四种基团中,名称和结构简式都正确的官能团是