The correct figure that shows,schematically,the wave pattern produced by the superposition of two waves of frequencies $9\,Hz$ and $11\,Hz$ is:

$A$ source of unknown frequency gives $4\, \text{beats/s}$ when sounded with a source of known frequency $250\, \text{Hz}.$ The second harmonic of the source of unknown frequency gives $5\, \text{beats/s}$ when sounded with a source of frequency $513\, \text{Hz}.$ The unknown frequency is .... $\text{Hz}$

The maximum number of beats per second that a human ear can distinguish is

An unknown frequency $x$ produces $8$ beats per second with a frequency of $250 \ Hz$ and $12$ beats per second with a $270 \ Hz$ source. Then $x$ is .... $Hz$.

Two sitar strings,$A$ and $B$,playing the note $'Dha'$ are slightly out of tune and produce beats at a frequency of $5 \, Hz$. The tension of string $B$ is slightly increased and the beat frequency is found to decrease to $3 \, Hz$. If the frequency of $A$ is $425 \, Hz$,the original frequency of $B$ is ... $Hz$.

It is possible to hear beats from two vibrating sources of frequency: