In multi-unit smooth muscles, the different muscle fibres of the group :

Options

A. Contract independently
B. Contract simultaneously
C. Do not contract at all
D. First contract and then stimulate other groups for contraction

Answer

### Step-by-Step Solution

1. **Understanding Multi-Unit Smooth Muscles**:
- Multi-unit smooth muscles are a type of smooth muscle found in various organs. Unlike single-unit smooth muscles, where cells are interconnected and contract as a single unit, multi-unit smooth muscles consist of individual muscle fibers that operate independently.

2. **Analyzing the Options**:
- **Option A**: Contract independently.
- **Option B**: Contract simultaneously.
- **Option C**: Do not contract at all.
- **Option D**: First contract and then stimulate other groups for contraction.

3. **Evaluating Each Option**:
- **Option A**: This option states that the muscle fibers contract independently. This is true for multi-unit smooth muscles, as each fiber can contract and relax on its own without influencing others.
- **Option B**: This option suggests that all fibers contract simultaneously, which is characteristic of single-unit smooth muscles, not multi-unit.
- **Option C**: This option claims that the fibers do not contract at all, which is incorrect as multi-unit smooth muscles are capable of contraction.
- **Option D**: This option implies that one fiber contracts and stimulates others to contract, which is also a feature of single-unit smooth muscles.

4. **Conclusion**:
- Based on the evaluation, the correct answer is **Option A**: the different muscle fibers of the group contract independently.