Answer
This is an arithmetic sequence. The common difference, $d$, is $-2$.
Work Step by Step
Let's look at the terms to see if we can find a pattern:
$a _2 - a_1 = -1 - 1 = -2$
$a_3 - a_2 = -3 - (-1) = -2$
$a_4 - a_3 = -5 - (-3) = -2$
$a_5 - a_4 = -7 - (-5) = -2$
This is an arithmetic sequence because the same number is added to the previous term to get the next term, which means there is a common difference, and that difference $d$ is $-2$.
