Explanation: A positive temperature coefficient in a crystal means that its resonant frequency changes in the *same direction* as the temperature. 1. If the temperature *increases*, the operating frequency will *increase*. This confirms statement B. 2. If the temperature *decreases*, the operating frequency will *decrease*. This confirms statement A. Therefore, both A and B accurately describe the behavior of a Y-cut crystal, which has a positive temperature coefficient. Statement C describes the characteristic of a crystal with a *negative* temperature coefficient, where frequency decreases as temperature increases, making it incorrect for this scenario. Hence, D (A & B) is the correct answer. This understanding is critical for selecting the right crystal for stable frequency generation in amateur radio equipment, as frequency stability is often dependent on temperature.