Changing the name of a cell by C not working

Hi,

I'd like to change the name of cells in Excel. I replaced A1 by A, A2 by B, A4 by D, but it doesn't work to replace A3 by C. When I enter C in the name of the cell, all the column A is selected but that's not what I did.

Why can't we name C a cell ? Do you know how could I do that ?

Cheers.