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];}