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According to the circuit diagram, the resistor R is connected in series with the resistor of the alcohol gas sensor, and the voltage across R is measured with a voltmeter.

When the sensor gradually approaches the top of the glass bottle containing alcohol, the voltage indication number gradually increases.

∴ According to Ohm's law, the resistance in the circuit becomes smaller, and the resistance of the alcohol gas sensor becomes smaller, that is, the resistance of the sensor gradually decreases, so C is correct and D is incorrect;

At this time, the voltage across the resistor R becomes larger,

The total voltage in the series circuit is equal to the sum of the sub-voltages,

∴ The voltage across the sensor decreases gradually.

So choose C.