三、完善程序（单选题，每小题3分，共计30分）

第1题

给定一个长度为 $n$、由正整数组成的序列 $a_1,a_2,\ldots,a_n$，请你求出所有子段中第 $k$ 小的子段和。

#include<iostream>
int n,k,a[100005];
bool check(long long mid) {
    long long s = 0;
    for(int l = 1,r = 1;r <= n; r++) {
        while(l <= r && ____(1)____) l++;
        s+=____(2)____;
    }
    return ____(3)____;
}
int main(){
    std::cin>>n>>k;
    for(int i=1;i<=n;i++) {
        std::cin >>a[i];
        a[i]+=a[i-1];
    }
    long long lbound = 0,rbound = 5000000000,mid;
    while(lbound <= rbound) {
        mid = (lbound + rbound) >> 1;
        if(check(mid)) ____(4)____rbound = mid - 1;
        else ____(5)____lbound = mid + 1;
    }
    std::cout << lbound;
    return 0;
}

(4)(5) 处应填（ ）。

{{ select(1) }}

• lbound = mid + 1、rbound = mid
• rbound = mid - 1、lbound = mid
• lbound = mid + 1、rbound = mid - 1
• rbound = mid - 1、lbound = mid + 1