Explanation: High coupling between the plate and grid circuits of a quartz-crystal oscillator means too much power is fed back to the crystal. Crystals are piezoelectric devices that vibrate mechanically at their resonant frequency. They have specific power handling limits. When driven excessively hard by strong feedback, the crystal vibrates with too much amplitude, generating internal friction and heat. This excessive heat can damage the crystal's delicate structure, shift its operating frequency, or even cause it to crack and break. Therefore, overheating and possible breakage (C) are direct consequences of overdriving the crystal due to high coupling. A) Grid-to-plate capacitance is a property of the tube/transistor, not destroyed by coupling strength. B) While higher coupling *could* increase oscillation amplitude, high Q refers to the crystal's inherent efficiency and narrow bandwidth, not a direct outcome of excessive coupling. The primary danger of over-coupling is damage, not just "higher Q." D) Overmodulation relates to excessive audio input in an AM transmitter, which is unrelated to the internal operation of a crystal oscillator's feedback loop.