His proposal is very _________. We do not need any explanation.[ ]

A. abstract                    

B. faithful

C. loyal                          

D. concrete

2000年，燃煤工业锅炉平均运行效率65%左右，比国际先进水平低( )个百分点。

A．16～21
B．15～20
C．17～22
D．15～19