普通低合金结构钢比普通碳素钢的（）。
A.强度低
B.强度高
C.抗冲击性差
D.加工难度大
参考答案：B
---- Did you visit the Big Ben in London?
---- No, we ______ it, but we spent too much time shopping.
A．could visit
B．could have visited
C．must ahave visited it
D．can have visited
重组DNA技术常包括哪些基本步骤？
