下面merge_sort函数试图实现归并排序算法，横线处应该填入的是（ ）。

>#include <vector>
using namespace std;
void merge_sort(vector<int> & apr, int left, int right) {
    if (right - left <= 1)
    return;
    int mid = (left + right) / 2;
    merge_sort(___,___) ; // 在此处填入选项
    merge_sort(___,___) ; // 在此处填入选项
    vector<int> temp(right - left);
    int i = left, j = mid, k = 0;
    while (i < mid && j < right)
    if (arr[i] <= arr[j])
    temp[k++] = arr[i++];
    else
    temp[k++] = arr[j++];
    while (i < mid)
    temp[k++] = arr[i++];
    while (j < right)
    temp[k++] = arr[j++];
    for (i = left, k = 0; i < right; ++i, ++k)
    arr[i] = temp[k];
}