Correct Answer: Option A

Explanation

Let the count rate at zero time be N 

N \(\frac{3 min}{\to}\)  \(\frac{N}{2}\) \(\frac{3 min}{\to}\) \(\frac{N}{4}\)

\(\frac{N}{4}\) 

= 400

N = \(\frac{400}{N/4}\) x N 

= 1600

There is an explanation video available below.

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