Two harmonic waves moving in the same direction superimpose to form a wave \( x = a \cos(1.5\, t)\cos(50.5\, t) \) where \( t \) is in seconds. Find the period with which they beat. (close to nearest integer):
\( 4 \, \text{s} \)
\( 2 \, \text{s} \)
\( 6 \, \text{s} \)
\( 1 \, \text{s} \)