Question:

<p>Let $r_1 = \frac{p_1}{q_1}$ and $r_2 = \frac{p_2}{q_2}$ be two rational numbers where $p_i, q_i \in \mathbb{Z}$ and $q_i \neq 0$.</p><p>Then $r_1 + r_2 = \frac{p_1}{q_1} + \frac{p_2}{q_2} = \frac{p_1q_2 + p_2q_1}{q_1q_2}$, which is also of the form $\frac{p}{q}$ where $p, q \in \mathbb{Z}$ and $q \neq 0$.</p><p>Therefore, the sum of two rational numbers is always rational. The answer is (a).</p>