The property of the electric line of force
a. Electric lines of force are just imaginary lines
b. Electric lines of force will be parallel to the surface of conductor
c. If the lines of force are crowded, them field is strong
d. Electric lines of force are closed loops

Options

A. both a and c
B. bothh b and d
C. only a
D. all

Answer

To solve the question regarding the properties of electric lines of force, we will analyze each option provided and determine which statements are true based on the principles of electric fields.

### Step-by-Step Solution:

1. **Understanding Electric Lines of Force**:
- Electric lines of force are conceptual tools used to visualize electric fields. They represent the direction and strength of the electric field created by electric charges.

2. **Analyzing Option A**:
- **Statement**: "Electric lines of force are just imaginary lines."
- **Evaluation**: This statement is true. Electric lines of force are indeed imaginary constructs that help us understand electric fields, as they cannot be seen directly.

3. **Analyzing Option B**:
- **Statement**: "Electric lines of force will be parallel to the surface of the conductor."
- **Evaluation**: This statement is false. Electric lines of force are always perpendicular to the surface of a conductor in electrostatic equilibrium. This is because the electric field inside a conductor is zero, and thus the lines of force cannot be parallel to the surface.

4. **Analyzing Option C**:
- **Statement**: "If the lines of force are crowded, then the field is strong."
- **Evaluation**: This statement is true. When electric lines of force are closer together (crowded), it indicates a stronger electric field in that region. The density of the lines correlates with the strength of the electric field.

5. **Analyzing Option D**:
- **Statement**: "Electric lines of force are closed loops."
- **Evaluation**: This statement is false. Electric lines of force do not form closed loops; they originate from positive charges and terminate at negative charges. They do not return to the same charge.

6. **Conclusion**:
- Based on the evaluations:
- Option A is true.
- Option B is false.
- Option C is true.
- Option D is false.
- Therefore, the correct answers are A and C.

### Final Answer:
The properties of the electric line of force are:
- A. Electric lines of force are just imaginary lines (True).
- C. If the lines of force are crowded, then the field is strong (True).