在压力容器焊接接头的表面质量中，（）缺陷是根据压力容器的具体情况而要求的。
A.肉眼可见的夹渣
B.未熔合
C.弧坑
D.咬边
参考答案：D
—English has a large vocabulary, hasn’t it?
—Yes. ________ more words and expressions and you’ll find it easier to read and communicate.
A．Know
B．Knowing
C．To know
D．Known
He talked about a hero ______no one had ever heard. [ ]
A. of whom
B. from whom
C. about that
D. who
