Why we can't take arm length as the standard unit of length?

Explanation

### Step-by-Step Solution:

1. **Understanding Standard Units**: A standard unit of measurement is a fixed quantity that everyone agrees upon. This ensures consistency and accuracy in measurements across different contexts and individuals.

2. **Variability of Arm Length**: Arm length varies from person to person. For example, one person may have longer arms than another. This variability means that if we used arm length as a standard unit, the measurement would not be consistent.

3. **Examples of Variability**: Consider a group of people: one person may have an arm length of 60 cm, while another may have an arm length of 75 cm. If we used arm length as a unit, the same physical distance could be measured differently by different people.

...