Which statement is correct :

Options

A. Iron rims are cooled before they are placed on cart wheels
B. A glass stopper gets tight on warming the neck of the bottle
C. Telephone wires sag in winter, but become tight in summer
D. A little space is left between two rails on a railway track.

Answer

To determine which statement is correct, we will analyze each statement one by one based on the principles of thermal expansion and contraction.

### Step-by-Step Solution:

1. **Analyze Statement A**: "Iron reams are cooled before they are placed."
- When iron is heated, it expands. To fit iron reams onto cartwheels, they are often cooled first. This cooling allows them to contract, making it easier to fit them onto the wheels. Therefore, this statement is correct.

2. **Analyze Statement B**: "A glass stopper gets tight on warming the neck of the bottle."
- When glass is heated, it expands. If the neck of the bottle is warmed, the glass stopper will also expand, potentially making it looser, not tighter. Thus, this statement is incorrect.

3. **Analyze Statement C**: "Telephone wires sag in winter but become tight in summer."
- In winter, the temperature drops, causing the wires to contract and sag. In summer, the temperature rises, causing the wires to expand and become tighter. This statement is correct.

4. **Analyze Statement D**: "A little space is left between two rails on a railway track."
- It is standard practice to leave a space between railway tracks to allow for thermal expansion. As temperatures increase, the rails expand, and the space helps prevent buckling. Therefore, this statement is also correct.

### Conclusion:
After analyzing all statements, we find that:
- Statements A, C, and D are correct.
- Statement B is incorrect.

### Final Answer:
The correct statements are A, C, and D.