Explanation: A quartz crystal provides exceptionally high master-oscillator frequency stability (A) due to its piezoelectric properties. When an alternating voltage is applied, the crystal vibrates mechanically at a precise resonant frequency. This mechanical resonance is extremely sharp, giving the crystal a very high "Q" factor compared to traditional LC tank circuits, which translates directly into superior frequency stability for the oscillator. Furthermore, a crystal's inherent mechanical resonance is so well-defined that it can act as the sole frequency-determining (tuned) circuit in an oscillator (B). In many crystal oscillator designs, the crystal itself replaces the inductor and capacitor combination that would typically form the tuned circuit, dictating the output frequency with great precision. Since both statements A and B accurately describe the function of a quartz crystal in a radio transmitter, option C is the correct choice.