You would use the Change event of the parent sheet object - test to see if the cells altered are in Column C and if so what their respective values are - in turn adjusting B as stated.
For a generic and totally untested example:
to insert the above right click on sheet to which the above is to be applied - select View Code and paste above into resulting window.
Thereafter in a macro enabled file altering value in C to "X" should adjust the string in B such that it appends the present value in A to it.
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