MS Access continuous form with dropdown that should show for each row different data depending on value of another field in same row

This is an Access 2013 VBA question.

I have a subform of type continuous form that shows selected rows of data from a table. in the first field I have a kind of record type, say A or B. In the third field, which is shown as a a combobox, I have a value that has to come from two different data sets depending on the record type. I.e.
- if record type is A, then the dropdown in the third field should show a set of values (selected from a separate table, in a column that holds the possible values of the 3rd field  for record type A)
- if record type is B,  then the dropdown in the third field should show a different set of values (selected from the same separate table, in a column that holds the values of the 3rd field  for record type B)

I am really stuck with that user requirement, and don't know if I should have
 - 1 or  2 comboboxes for the 2 record types and ony show one or the other
- 2 separate select statements depending on the type
- and even how to find out, when the row is displayed, if record type is or B.

There doesn't seem to be an event that I can catch when each row is being displayed, so that I could populate the values for the combobox(es) properly depending on the record type

Any idea how I can do what the users want ?

Thanks for help.
Bernard