小杨编写了一个如下的高精度乘法函数，则横线上应填写的代码为（ ）。
1 vector<int> multiply(vector<int>& a, vector<int>& b) {
2 int m = a.size(), n = b.size();
3 vector<int> c(m + n, 0);
4
5 // 逐位相乘，逆序存储
6 for (int i = 0; i < m; i++) {
7 for (int j = 0; j < n; j++) {
8 c[i + j] += a[i] * b[j];
9 }
10 }
11
12 // 处理进位
13 int carry = 0;
14 for (int k = 0; k < c.size(); ++k) {
15 ________________________________ // 在此处填入代码
16 c[k] = temp % 10;
17 carry = temp / 10;
18 }
19
20 while (c.size() > 1 && c.back() == 0)
21 c.pop_back();
22 return c;
23 }
int temp = c[k];
int temp = c[k] + carry;
int temp = c[k] - carry;
int temp = c[k] * carry;