单项选择题

若有下列说明:
int a[12]=1,2,3,4,5,6,7,8,9,10,11,12;
char c='a',d,g;
则数值为4的表达式是 ______。

A.a[g-c]
B.a[4]
C.a['d'-'c']
D.a['d'-c]