A brake warning lamp on the dash remains on whenever the ignition is on. If the wire to the pressure differential switch (usually a part of a combination valve or built into the master cylinder) is unplugged, the dash lamp goes out. Technician A says that this is an indication of a hydraulic brake problem. Technician B says that the problem is probably due to a stuck parking brake cable switch. Which technician is correct?

The answer is Technician A only is correct.

Explanation:

From the statement that a brake warning lamp on the dash remains usually on whenever the ignition is on, thus if the wire to the pressure differential switch (usually a part of a combination valve or built into the master cylinder) is unplugged, the dash lamp goes out. This is an indication of a hydraulic brake problem because the wire to the brake switch is disconnected thus causing no reportage. Normally, the brake warning lamp supposed to be on  usually except if faulty or wire is disconnected as described above.

d) the number of neutrons plus the number of protons