Which graph does NOT represent sequence graph?

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I have 4 graphs and should choose which ones are NOT sequence graphs.

enter image description here

The first is a constant sequence graph, eg. a(n)= 2.

The second is not a sequence graph, because it is a sinus function.

The third is a sequence graph, eg. a(n) = -3+n.

The fourth - I am not sure, but I think it is not.

Thx for reply.