已知 char a[4] = "ABC", b[4] = "123"; 下列选项正确实现字符串拼接输出 “ABC123” 的是（ ）

strcpy(a, b); cout << a << endl;

char c[4] = ""; strcpy(c, a); strcpy(c, b); cout << a << endl;

a += b; cout << a << endl;

char d[8] = ""; strcat(d, a); strcat(d, b); cout << d << endl;