数组 a 和 b 按低位在前的顺序保存两个非负大整数。下面代码实现高精度加法，横线处应填写 （　　）。

vector<int> add(const vector<int> &a, const vector<int> &b) {
    vector<int> c;
    int carry = 0;
    int n = max(a.size(), b.size());

    for (int i = 0; i < n; i++) {
        int sum = carry;
        if (i < a.size())
            sum += a[i];
        if (i < b.size())
            sum += b[i];
        c.push_back(sum % 10);
        ____________________
    }
    if (carry)
        c.push_back(carry);
    return c;
}

{{ select(1) }}

• carry = sum % 10;

• carry = sum;

• carry = sum / 10;

• carry = c[i] / 10;