We can do it in O(n).
It is similar to the MERGE procedure, where we merge two sorted arrays into one sorted array. The difference is that now array B is in reverse order. So, instead of starting indexing B from 0 and incrementing its index whenever we insert an element from B to C, we can start indexing B from n-1, and keep decrementing it whenever we insert an element from B to C, until it reaches 0.
Just some small changes to MERGE procedure will do the work.