View Compilation Error

0_0_20047116_2695.c: In function 'main':
0_0_20047116_2695.c:9:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
  for (int i = 1; i <= t; i++)
  ^
0_0_20047116_2695.c:9:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
0_0_20047116_2695.c: In function 'sum':
0_0_20047116_2695.c:24:2: error: expected expression before '/' token
  //return s;
  ^