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 numbers shown above is an arithmetic sequence,which of the following must also 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 and 2

e 2 and 3

answer: d

只有一个问题，从答案的解释上来看，题目说的是等差数列，但是我highlight的部分翻译过来怎么不是说等差数列的意思呢。。不知道她说的什么。。有谁能解释下么？？