$7)$ Two timpani $($tunable drums$)$ are played at the same time. One is correctly tuned so that when it is struck, sound is produced that has a wavelength of $2\mathrm{.}20$ m$.$ The second produces sound with a wavelength of $2\mathrm{.}10$ m$.$ If the speed of sound is $343$ m$/$s$,$ what beat frequency is heard?A$)3$ beats$/$sB$)5$ beats$/$sC$)7$ beats$/$sD$)13$ beats$/$sE$)15$ beats$/$s