下列C++程序实现了输出杨辉三角形，代码中横线部分应该填入的是（ ）。
#include <iostream>
using namespace std;
#define N 35
int a[N][N];
int main() {
int n;
cin >> n;
for (int i = 1; i <= n; i++)
for (int j = 1; j <= i; j++) {
if (j == 1 || j == i)
a[i][j] = 1;
else
__________ // 在此处填入选项
}
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= i; j++)
cout << a[i][j];
cout<<endl;
}
return 0;
}
a[i][j] = a[i - 1][j - 1] + a[i - 1][j];
a[i][j] = a[i][j - 1] + a[i - 1][j];
a[i][j] = a[i - 1][j] + a[i - 1][j];
a[i][j] = a[i - 1][j - 1] + a[i][j];