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> & arr, 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];
}

A.

arr, left, mid
arr, mid, right

B.

arr, left, mid + 1
arr, mid + 1, right

C.

arr, left, mid
arr, mid + 1, right

D.

arr, left, mid + 1
arr, mid + 1, right + 1

{{ select(1) }}

• A
• B
• C
• D