常规胸片，对哪种结构显示最差（）
A.肋骨
B.肋软骨
C.锁骨
D.横膈
E.胸椎
参考答案：B
After visiting Harbin, most of the foreign friends said they would never forget the time ________ they had spent.
A．that
B．what
C．when
D．at which
（）不属于BQ分级中岩石坚硬程度的类别。
A、坚硬岩
B、中硬岩
C、软岩
D、极软岩
