(2) The voltmeter V 1 and L 1 form a box, and the voltmeter V 1 measures the voltmeter at both ends of L 1; Voltmeter V2 and power supply form a box. Voltmeter V2 measures the voltage across the power supply.
The voltmeter V 1 measures the voltmeter at both ends of L 1, and the voltmeter V2 measures the voltage at both ends of the power supply, but the switch cannot control the voltmeter.
B voltmeter V 1 measures the voltmeter at both ends of L 1, voltmeter V2 measures the voltage at both ends of the power supply, and the switch controls the whole circuit.
C voltmeter V 1 measures the voltmeter at both ends of L 1, and voltmeter V2 measures the voltage at both ends of L2.
D voltmeter V 1 measures the voltmeter at both ends of the measuring power supply, and voltmeter V2 measures the voltage at both ends of L 1.
So choose B.