In order to solve this exercise you need to remember the following: In an Arithmetic sequence, there next consecutive term is obtained by adding or subtracting a constant. This constant is called "Common difference".
In this case, you have the following sequence:

Let's check it they have a common difference applying the procedure shown below:

You can notice that there is not a Common difference between the consecutive terms. Therefore, it is not an Arithmetic sequence.
The answer is: It is not an Arithmetic sequence, because there is not Common difference between the consecutive terms.