Answer
$(B). 1700$
Work Step by Step
We know that
$f=\frac{T-39}{13s}$
$\implies f_{75}=\frac{75-39}{13s}=2.77Hz$
similarly $f_{63}=\frac{63-39}{13s}=1.85Hz$
Now, the average frequency is
$\frac{2.77+1.85}{2}=2.31Hz$
The number of chirps can be calculated as
$N=ft_{total}$
$\implies N=(2.31)(12)(\frac{60s}{1min})$
$N=1700chirps$
