单项选择题
A.0.75/0.50/2.5Δ B.0.50/0.75/4Δ C.0.50/0.750/2.5Δ D.0.75/0.50/4Δ
A.仅仅是a B.仅仅是c C.a+b D.a+c
A.a+b B.a~c C.a~d D.a+b+c+d