源皮距增加可以（）
A.提高皮肤剂量
B.提高空气剂量
C.降低穿透力
D.提高深度量
E.以上都不是
参考答案：D
I don't think Richard could have done such a stupid thing last night, _____? [ ]
A. do I
B. could he
C. did he
D. has he
用一根10.28米长的绳子围一个半圆形，这个半圆的直径是______米．
