Define the S.I. unit of three fundamental quantities.

Explanation

To define the S.I. unit of three fundamental quantities, we will follow these steps:

### Step 1: Understand the S.I. System
The S.I. system, or the International System of Units, is the modern form of the metric system. It is the official system of measurement in nearly every country around the world.

### Step 2: Identify Fundamental Quantities
In physics, there are seven fundamental physical quantities. These are:
1. Length
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