A single fixed pulley is used because it :

Options

A. has a mechanical advantage greater than 1.
B. has a velocity ratio less than 1.
C. gives 100% efficiency.
D. helps to apply the effort in a convenient direction.

Explanation

To determine why a single fixed pulley is used, we need to analyze the options provided in the question.

### Step-by-Step Solution:

1. **Understanding Mechanical Advantage**:
- A single fixed pulley has a mechanical advantage (MA) of 1. This means that the force you exert (effort) is equal to the load you lift. Therefore, the first option stating that it has a mechanical advantage greater than one is incorrect.

2. **Understanding Velocity Ratio**:
- The velocity ratio (VR) of a single fixed pulley is also 1. This means that for every unit of distance the effort moves, the load moves the same distance. Thus, the second option claiming that it has a velocity ratio less than one is also incorrect.

3. **Considering Efficiency**:
- The efficiency of a machine is defined as the ratio of useful work output to the total work input. In the case of a single fixed pulley, there can be friction between the pulley and the rope, which means it cannot achieve 100% efficiency. Therefore, the third option stating that it gives 100% efficiency is incorrect.

4. **Convenient Direction of Effort**:
- The primary advantage of a single fixed pulley is that it allows the user to apply effort in a more convenient direction. Instead of lifting the load directly upwards, the user can pull down on the rope, which can be easier and more comfortable. Hence, the fourth option stating that it helps to apply the effort in a convenient direction is correct.

### Conclusion:
The correct answer is that a single fixed pulley is used because it helps to apply the effort in a convenient direction.

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