Indicate whether the following statements are true (t) or false (f). 1. the difference of two integers is always a natural number. 2. the difference of two integers is always an integer. 3. the sum of two integers is always an integer. 4. the quotient of two integers is always an integer (provided the denominator is non-zero). 5. the ratio of two integers is always positive 6. the product of two integers is always an integer. 7. the quotient of two integers is always a rational number (provided the denominator is non-zero).