Concept:
For two waves of frequencies \(f_1\) and \(f_2\),
\[
\text{Beat frequency}
=
|f_1-f_2|.
\]
The number of beats per minute is
\[
60\,|f_1-f_2|.
\]
Step 1: Determine the frequencies of the two waves.
Comparing
\[
y_1=2\sin(500\pi t-ax)
\]
with
\[
y=A\sin(\omega t-kx),
\]
we get
\[
\omega_1=500\pi.
\]
Hence,
\[
f_1=\frac{\omega_1}{2\pi}
=\frac{500\pi}{2\pi}
=250\,\text{Hz}.
\]
Similarly,
\[
\omega_2=506\pi.
\]
Therefore,
\[
f_2=\frac{\omega_2}{2\pi}
=\frac{506\pi}{2\pi}
=253\,\text{Hz}.
\]
Step 2: Calculate the beat frequency.
\[
f_b
=
|f_2-f_1|.
\]
\[
f_b
=
|253-250|.
\]
\[
f_b=3\,\text{Hz}.
\]
Step 3: Find the number of beats per minute.
\[
N
=
60\times3.
\]
\[
N=180.
\]
Step 4: Write the final answer.
\[
\boxed{180}
\]
\[
\boxed{\text{Answer = (B)}}
\]