The correct answer is:
2. The sum of an irrational and rational number can't be predicted.
This choice aligns with the idea that if you add a rational number (which can be expressed as a fraction) to an irrational number (which cannot be expressed as a fraction), the result will always be irrational. This is because if the result were rational, it would imply that the irrational number could be expressed as a rational number, which contradicts the definition of an irrational number.
Thus, the closure property implies that rational and irrational numbers behave in a way that leads to the sum being irrational.