You would have to do this using DCPs. They allow you to change the second prompt based upon what is entered for the first prompt.

I see now. In that case you will have to get a little tricky and create a Command object that links the table another table which lists the possible selections. Join the two tales on field B so that in the second table only the fields you want to appear will show up because they are the ones that matched the value in Col B.

Not sure how much sense that made, but not sure how else to say it right now. Basically, if you join a table of your selections based on Col B, then only the selections for the current value of B will be shown.