When skeletal muscle contraction occurs :

Options

A. the H and I bands gets smaller
B. the Z lines moves closer together
C. the width of A bands remain constant
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding Skeletal Muscle Contraction**:
- Skeletal muscle contraction occurs through a process known as the sliding filament theory. This theory explains how muscles contract at the microscopic level.

2. **Sliding Filament Theory**:
- According to the sliding filament theory, during contraction, the thin filaments (actin) slide over the thick filaments (myosin). This results in the shortening of the muscle fiber.

3. **Changes in Bands During Contraction**:
- As the muscle contracts, the following changes occur:
- **H-zone**: The H-zone, which is the area of the thick filament that does not overlap with thin filaments, becomes smaller.
- **I-band**: The I-band, which is the area of thin filaments that does not overlap with thick filaments, also becomes smaller.

4. **Z-lines Movement**:
- The Z-lines, which mark the boundaries of a sarcomere (the basic unit of muscle contraction), move closer together as the muscle contracts. This is due to the sliding of the filaments.

5. **A-band Width**:
- The A-band, which represents the length of the thick filaments (myosin), remains constant in width during contraction. This is because the thick filaments do not change in length; they simply slide past the thin filaments.

6. **Conclusion**:
- Therefore, during skeletal muscle contraction:
- The H-zone and I-band get smaller.
- The Z-lines move closer together.
- The width of the A-band remains constant.
- All these statements are correct, making the answer: **All of these**.

### Final Answer:
The correct answer is option number four: **All of these**.

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