The common difference, d, is 5 and the starting value, a, is 2.
Filling this in the form of

<span>you get
</span>
Answer:
D.
Step-by-step explanation:
There are 12 such shapes. One can start with two squares, adding squares around the edge and rejecting shapes that have been previously seen. Repeating until you have 5 squares results in 12 distinct shapes.
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2 shapes are possible with 3 squares: L, I
5 shapes are possible with 4 squares: L, I, N, T, O