Graves' disease is due to an autoantibody that binds to and activates the TSH receptor.This interaction would primarily increase which second messenger in the thyroid epithelial cells?

A)Ca⁺⁺
B)Activated signal transducer and activator of transcription (STAT)
C)Phosphatidylinositol 3-kinase (PI3K)
D)Cyclic guanosine monophosphate (cGMP)
E)Cyclic adenosine monophosphate (cAMP)