下列归并排序的分治部分，横线处应填入：
void merge_sort(int a[], int left, int right) {
if (left >= right) return;
int mid = (left + right) / 2;
merge_sort(a, left, mid);
________; // 在此处填入选项
merge(a, left, mid, right);
}
- A. merge_sort(a, mid, right)
- B. merge_sort(a, mid + 1, right)
- C. merge_sort(a, left, mid + 1)
- D. merge_sort(a, mid - 1, right)
正确答案：B