沈括《梦溪笔谈》载:“世间锻铁所谓钢铁者,用柔铁屈盘之,乃以生铁陷其间,泥封炼之,锻令相入,谓之团钢。”这项技术最早出现于[ ]
A.春秋时期
B.西汉
C.南北朝
D.北宋
答案:C
It’s years since I _______ him last time, and I haven’t written back to him.
A.heard of
B.heard out
C.heard about
D.heard from
钢结构用钢主要是热轧成形的钢板和型钢等,钢板最小厚度大于()mm为厚板,主要用于结构。
A.3
B.4
C.5
D.6