A geometric sequence is one in which the ratio of any term after the first to the preceding term is a constant. If the letters a, b, c, d represent a geometric sequence in normal alphabetical order, which of the following must also represent a geometric sequence for all values of k?
I. dk,ck,bk,ak
II.a+k,b+2k,c+3k,d+4k
III.$$ak^{4},bk^{3},ck^{2},dk$$