能延缓砼凝结时间，对砼后期强度发展无不利影响的外加剂是（）
A.早强剂
B.减少剂
C.引气剂
D.缓凝剂
参考答案：D
English is a language shared by several diverse cultures, each of _____ uses it somewhat differently. [ ]
A. which
B. what
C. them
D. those
洗舱作业时，人员下舱作业的要求是（）。
A.可燃气体浓度≤LEL2%
B.氧气浓度≥21%
C.可燃气体浓度≤2%，氧气浓度≥21%
D.可燃气体浓度≤LEL2%，氧气浓度≥21%
