Changing answers on scantron - carbon copy Help!!!

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Hi,

Hope everyone did well on the exam. I sat in the Hong Kong exam and it was jam packed.

Anyways, I for one am quite worried because I totally forgot about the carbon copy underneath the scantron sheet and was erasing/changing my answers like I normally would with a regular scantron.

What is the purpose of the carbon copy underneath the scantron?
Are my marks based on the scantron or the carbon copy??

Thanks for everyone's input and good luck!!