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 be an arithmetic sequence?

1) 2p, 2r, 2s, 2t, 2u
2) p-3, r-3, s-3, t-3, u-3
3) p^2, r^2, s^2, t^2, u^2

A) 1 only
B) 2 only
C) 3 only
D) 1 & 2
E) 2 & 3

pls explain in detail