P, r, s, t, u
An arithmetic sequence is a sequence in which each term after the first is equal to the sum of the preceding term and a constant. If the list of letters shown above is an arithmetic sequence, which of the following must also be an arithmetic sequence?
I 2P, 2r, 2s, 2t, 2u
II P-3, r-3, s-3, t-3, u-3
III $${P}^{2}$$, $${r}^{2}$$, $${s}^{2}$$, $${t}^{2}$$, $${u}^{2}$$