Define elementary physical quantities .

Explanation

### Step-by-Step Solution

1. **Definition of Elementary Physical Quantities**:
Elementary physical quantities are the basic physical quantities that cannot be derived from other quantities. They are fundamental in nature and serve as the building blocks for other derived physical quantities.

2. **Examples of Elementary Physical Quantities**:
- **Mass (m)**: It is represented by the symbol 'm' and is a measure of the amount of matter in an object. Its dimension is simply [M].
- **Length (l)**: Represented by the symbol 'l', it is the measure of distance. Its dimension is [L].
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