The indication of ammeter A becomes zero, indicating that the circuit fault is an open circuit; When conducting parallel detection with wires and electrical appliances, if the ammeter displays numbers after the wires and electrical appliances are connected in parallel, it means that the electrical appliances connected in parallel with wires are open; If there is no indication, another electrical appliance is open or both electrical appliances are open.

A, parallel wires are connected at both ends of R, and the ammeter has no indication. It may be that L is open, or both R and L are open, so A is wrong;

B, connect the wires in parallel at both ends of L, and the ammeter has no indication. Maybe R is on, or both R and L are on, so B is wrong;

C, connect the wires in parallel at both ends of R, and the ammeter displays the numbers. It must be R, that's right;

D. Connect the wires in parallel at both ends of L, and the ammeter displays the numbers. L must have opened the way, so d is wrong.

So choose C.