--- I’m afraid I’ve failed the driving test.

--- ______. Maybe things are not so bad as you think.

A．That’s OK

B．I think so

C．Take it easy

D．It’s very kind of you

关于X线在物质中衰减的叙述，正确的是

A．X线衰减与X线波长无关

B．X线衰减与透过物质有关

C．线性衰减系数是以长度为单位的衰减

D．质量吸收系数是以质量厚度为单位的衰减

E．总衰减系数是各种衰减系数的总和