设有如下定义,unsigned long a[100] = {0, 1, 3, 9};unsigned long *pa = a, *pb = &a[3], *pc;int x;则下面语句中存在语法错误的是(A)x = pb - pa; (B)pc = pb - 2;(C)pc = pa + pb; (D)pc = pb + (pb - pa);