When are angle cocks allowed to be left partially opened or closed?

Answer: C — Never

Angle cocks should never be left partially opened or closed because this can create dangerous situations and lead to improper functioning of the air brake system. Angle cocks are critical components in controlling the flow of air between cars in a train. If they are not fully opened or fully closed, it can result in incomplete braking or unintended air losses, increasing the risk of accidents or ineffective braking during operation. Therefore, for safety and operational integrity, angle cocks must be fully in one position or the other, ensuring that the air brake system functions as intended.