A longitudinal wave of frequency 1000 Hz is travelling in a medium . At a certain point in the medium there is a compression at certain moment . How many milliseconds later would the next rare faction form at this place ?

Question

$(a)\;2\;milliseconds\qquad(b)\;0.5\;milliseconds\qquad(c)\;1\;milliseconds\qquad(d)\;4\;milliseconds$