Two sound waves of wavelengths `lambda_1` and `lambda_2` `(lambda_1 > lambda_2)` produce n beats/sec, speed of sound is
`1)` `n(lambda_2 - lambda_1)`/`(lambda_1 lambda_2)`
`2)` `n(lambda_2 - lambda_1)`
`3)` `n(lambda_1 lambda_2)`/`(lambda_2- lambda_1)`
`4)` `n(lambda_1 lambda_2)`/`(lambda_1 + lambda_2)`