I have an excel sheet with the following values

A B
1 1a
1 1b
2 2a
2 2b
3 3a


Now if i try to enter a value in an empty cell...i should get a
dropdown containing the distinct values from the column -A....and when
a value is selected..then when i try to enter a new value in the next
cell i should get a dropdownlist containing the values in column
B..but the one that are related to column A only

ie if i select 1 then the next cell should have only 1a and 1b