Which graph shows a periodic function with an amplitude of 2?

For this case, the first thing you should notice is that they are repeated because they have values that are repeated.
 Then, we should observe the highest and lowest peak of each graph.
 We notice that the second graph has amplitude equal to two.
 We have:
 A = (l-4l + l0l) / 2
 A = 4/2
 A = 2
 
 second graph

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