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

**Step 1: Define the Activity of a Radionuclide**
The activity of a radionuclide is defined as the number of radioactive disintegrations occurring per unit time in a given sample. Mathematically, it can be expressed as:

\[ A = -\frac{dN}{dt} \]
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