(20 13? Jiyuan No.2 model) is a natural gas leakage detection circuit designed by four students of the science and technology team, as shown in the figure, where R is gas sensitive resistance (represented by symbols). A. According to the circuit diagram, R and R0 are connected in parallel, and the voltage of power supply is measured by voltmeter. Because the power supply voltage is constant, the gas concentration increases, and the resistance of the gas sensor decreases, the indication of the voltmeter remains unchanged, so it does not meet the question.

B, according to the circuit diagram, R and R0 are connected in parallel, and the current of R0 branch is measured by ammeter. According to the fact that parallel circuits work independently and do not affect each other, it can be seen that when the concentration of natural gas increases, the resistance of gas sensor decreases and the indication of ammeter remains unchanged, so it does not meet the question.

C. According to the circuit diagram, R and R0 are connected in series, and the ammeter measures the current in the circuit. When the concentration of natural gas increases, the resistance of the gas sensor decreases and the total resistance of the circuit decreases. According to ohm's law, the current in the circuit increases, that is, the indicated value of the ammeter increases, which accords with the meaning of the question.

D. According to the circuit diagram, R is connected in series with R0, and the voltmeter measures the voltage across R. When the concentration of natural gas increases, the resistance of the gas sensor decreases and the total resistance of the circuit decreases. According to the voltage dividing characteristics of series circuit resistors, it can be seen that the voltage across R decreases, that is, the indicated value of voltmeter decreases, so it does not meet the question.

So choose C.