The correct answer is: A. CST-9 sleeper
CST-9 sleepers are the most preferred type of sleepers on joints because they are made of concrete, which is a strong and durable material. They are also designed to be flexible, which helps to absorb the shock of train traffic and prevent the track from becoming damaged.
Steel trough sleepers are also made of a strong material, but they are not as flexible as CST-9 sleepers. This can cause the track to become damaged over time.
Wooden sleepers are not as strong as concrete or steel sleepers, and they are also not as durable. This means that they need to be replaced more often, which can be costly.
Concrete sleepers are the most expensive type of sleeper, but they are also the most durable. This means that they will last the longest and will not need to be replaced as often.
In conclusion, CST-9 sleepers are the best type of sleeper to use on joints because they are strong, durable, and flexible.