View Compilation Error

1_714_944_10008.c: In function 'main':
1_714_944_10008.c:7:4: error: expected expression before '/' token
    // freopen("test.txt","r",stdin);
    ^
1_714_944_10008.c:15:9: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
         for(long i=0;i<n;i++)
         ^
1_714_944_10008.c:15:9: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code