Serveral blocks of magnesium are fixed to the bottom of a ship to

Options

A. Prevent aciton of water and salt
B. Prevent puncturing by under sea rocks
C. keep away the sharks
D. Make the ship lighter

Explanation

To solve the question regarding why several blocks of magnesium are fixed to the bottom of a ship, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Properties of Magnesium**:
- Magnesium is a lightweight metal. This property is crucial for applications in shipbuilding, as reducing weight can enhance fuel efficiency and overall performance.

2. **Consider the Affinity for Oxygen**:
- Magnesium has a high affinity for oxygen, which means it can form a protective oxide layer when exposed to air. This oxide layer can help protect the underlying metal from corrosion, making it suitable for marine environments.

3. **Evaluate Toughness**:
- Magnesium is known for its toughness. This means it can withstand impacts and stresses that might occur in a marine environment, such as collisions with underwater objects.

4. **Analyze the Options Given**:
- The options provided in the question include:
- Preventing action of water and salt
- Preventing puncturing by undersea rocks
- Keeping away sharks
- Making the ship lighter
- Among these options, the one that aligns with the properties of magnesium is that it makes the ship lighter.

5. **Conclusion**:
- Therefore, the correct answer to the question is that magnesium blocks are fixed to the bottom of the ship primarily because they make the ship lighter.

### Final Answer:
The correct option is D: "makes the ship lighter."