Explanation: Quartz crystals operate on the principle of the piezoelectric effect, vibrating at a precise mechanical resonant frequency to generate a stable electrical signal. For proper operation, this resonance must be unimpeded and consistent. Any foreign material, such as dirt, grease, or even moisture, on the crystal's surface acts as an additional mass loading. This changes the crystal's mechanical resonance, shifting its operating frequency, reducing its Q factor (quality factor), and potentially preventing it from oscillating altogether. * **A) Not touching the faces of the crystal with the fingers:** This is a crucial preventative measure. Fingers naturally transfer oils, dirt, and skin cells, which can immediately contaminate the delicate crystal surface. * **B) Cleaning the faces with soap and water:** Soap is a surfactant that can emulsify and remove many types of dirt and grease, especially water-soluble contaminants. Thorough rinsing and drying are essential afterward. * **C) Cleaning the faces with carbon tetrachloride:** Carbon tetrachloride is a strong solvent for greases and oils (though less commonly used now due to toxicity and environmental concerns, other solvents like isopropyl alcohol serve a similar purpose). It effectively removes non-polar contaminants that soap and water might not fully address. Since all these actions (prevention, and two different cleaning methods) are valid strategies to ensure the crystal surface remains free of contaminants and operates correctly, "All of the above" is the comprehensive correct answer.