If then with last then being a drop down box with multiple selections

[lscott214]

Hello
I need one cell to hold this formula:

=if(a2="A","Yes",if(a2="B","No",if(a2="C",(dropdownlist with 3 values to select from)

When a2 = C then the drop down would be available and if the user selects one of the 3 options but changes a2 to B it does not overwrite the formula.