低碳钢通过（）热处理可以改善切削加工性。
A.正火
B.淬火
C.退火
参考答案：A
This is the first time I ______, so I feel very curious about everything here. [ ]
A. have been to this city
B. had been to this city
C. came to this city
D. come to this city
多层建筑施工，在计划安排上确须进行立体交叉作业时，应事先采取（）防护措施。
A.有效
B.防范
C.隔离
D.安全
