The correct answer is: C. Stop signal
A block instrument is a device used to control the movement of trains on a railway track. It consists of two signals, one at each end of the block, which are connected by a cable. When the block instrument is working properly, the signals will be in the “stop” position when there is a train in the block, and in the “clear” position when the block is clear.
If the block instrument becomes defective, the signals will not be able to communicate with each other. This means that the signals may be in the “clear” position even though there is a train in the block, or they may be in the “stop” position even though the block is clear. This can lead to a collision between trains.
To prevent this, all stop signals on a double line are treated as defective if the block instrument becomes defective. This means that trains must stop at all stop signals, even if the signals are in the “clear” position. This ensures that there is no risk of a collision.
Option A is incorrect because the first stop signal is not necessarily the only stop signal on a double line. There may be multiple stop signals on a double line, and all of them must be treated as defective if the block instrument becomes defective.
Option B is incorrect because the last stop signal is not necessarily the only stop signal on a double line. There may be multiple stop signals on a double line, and all of them must be treated as defective if the block instrument becomes defective.
Option D is incorrect because all stop signals on a double line are treated as defective if the block instrument becomes defective. This means that there is no “none of these” option.