ETL in Prime Numbers problem

Code is working but it exceeds time limit, i don’t have any idea how to make it faster.

Link to the problem: https://www.spoj.com/problems/PRIME1/3

In worst case scenario (i.e. t=10,m=999900000,n=1000000000) it takes less than 2s to run the program so i really don’t know what is the issue.

Here is the code:

#include < iostream >

using namespace std;

int main()
{
int t,m,n;

cin>>t;

for(int i=0; i<t; i++)
{
    cin>>m>>n;
    if(m<2) m=2;

    bool a[n+1];

    for(int j=2; j*j<=n; j++)
        {
            if(a[j]==0)
                {
                for(int k=j*j; k<=n; k=k+j)
                    a[k]=1;
                }
        }

    for(int j=m; j<=n; j++)
    {
        if(a[j]==0)
        {
            cout<<j<<endl;
        }
    }
    cout<<endl;
}

return 0;

}