防止混凝土早期冻害的措施包括（）。
A.提高混凝土早期强度
B.掺用缓凝剂
C.掺用引气剂
D.掺用早强型减水剂
参考答案：A, B, D
— I rang you at about nine, but there was no reply.
— Oh, that was probably________I was seeing the doctor. [ ]
A. why
B. when
C. what
D. that
