Unwanted letters next to tickboxes in Excel

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Occasional Visitor

Office 365 - Excel 0n Windows 10.  I have a questionnaire with multiple tickboxes.  Just re-opened my document - ALL of the tickboxes have gained the letter 'C' next to them.  I didn't put them there.  Why are they there and how to I get rid of them?

 

Cheers,

 

Mike