Answer
(a) equal to
(b) I
Work Step by Step
(a) We know that the beat frequency of the first two tones is $245Hz-240Hz=5Hz$. Similarly, the beat frequency for the second two tones is given as $145Hz-140Hz=5Hz$. Thus, the beat frequencies of the two set of tones is equal.
(b) We know that the correct answer is (I) -- that is, the beat frequency is determined by the difference in frequencies and is independent of their actual values.
