Read the following article and answer the question by choosing from the options.

It is like suddenly being super strong LOL . The feeling wears off after a while and I imagine it is just the nervous system adjusting after swing the heavy object around . What I ca n't get over is how hard the work is for my core .

What may be the reason they know it 's hard work for their core ?
Available choices:
[A]. None of the above choices ..
[B]. Because someone told them it is ..
[C]. Because they are feeling stronger ..
[D]. Because their appetite has increased .....A:
[A].