I got an answer:

The code to set the row source for the 2nd combo box should be in the After Update event of the 1st combo box.

This line should do the trick:

Me.NameOfSecondComboBox.Requery

Open the form in design view. Get the properties of combo box A. On the Events page, click inside the text box of the After Update row. Then click the ... button at the end of the row. The code window should open and you should be "inside" the correct event. Then just type in the code line that I sent you earlier, changing the names to suit, of course.