Conditional Drop Down List

[cy1313]

I have made a simple drop down list with a list a values contained on the
current spreadsheet. How do I make it so that each value can only be
selected once and after that value has been selected in one drop down list,
it is unavailable in the next drop down list?

 

[T. Valko]

See this:

http://contextures.com/xlDataVal03.html