Define the activity of a radionuclide. Write its SI unit. Give a plot of the activity of a radioactive species versus time.

Explanation

### Step-by-Step Solution

1. **Definition of Activity**:
- The activity of a radionuclide refers to the rate at which the nuclei of a radioactive sample decay. It quantifies how many disintegrations (decays) occur in a given time period. Mathematically, it can be expressed as:
\[ A = \lambda \cdot N \]
where:
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