In the circuit shown in the figure, the power supply voltage remains unchanged. When the switch S1 is closed, the lamp l will light up. If the switch S2 is closed again, then () A. The number of two ammeters increases B. the brightness of light l decreases C. The total resistance of the circuit decreases D. the number of voltmeters increases
When the switches are all closed, it is a parallel circuit, so the branch current remains unchanged and the total current becomes larger, so option a is incorrect; because the branch current remains unchanged, the brightness of lamp L remains unchanged, so option B is incorrect; in the parallel circuit, the more branches, the smaller the total resistance, so option C is correct; the voltage at both ends of the parallel circuit remains unchanged, so the voltage expression remains unchanged; so option C is selected