How do you solve this algorithm problem?

Suppose that you are given a sequence of n elements to sort. The input sequence
consists of n/k subsequences, each containing k elements. The elements in a given
subsequence are all smaller than the elements in the succeeding subsequence and
larger than the elements in the preceding subsequence. Thus, all that is needed to
sort the whole sequence of length n is to sort the k elements in each of the n/k
subsequences. Show an �Ω(n lg k) lower bound on the number of comparisons
needed to solve this variant of the sorting problem. (Hint: It is not rigorous to
simply combine the lower bounds for the individual subsequences.)

Solve this using adversary argument.