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;