Почему бактерии невидимые?

1. for i := 1 to 10 do begin readln(n); if (even(n)) then s := s + n; end;
2. for i := 1 to 15 do begin readln(n); if (n mod 3 = 0) and (n mod 5 = 0) then inc(c); end;
3. for i := 1 to 10 do begin readln(n); if (10 <= n) and (n <= 20) then m := m * n; end;