Clearing out data validation lists when another changes?

I am trying to clear multiple validation lists if one of two particular lists changes, but can't find the code to do this.

Basically, I want the following cells (which are each independent validation lists) to reset back to being 'blank' (B14,B15,B16,B17,B18,B19,B20,B21,B22,B26,B27,B28,B29,B30,B31,B32,B33,B34,B35,B36,B37) if either of two cells change (B20,B21).

If B20 changes, I also need it to 'reset' B21 to blank - but if B21 changes, B20 should not reset.

I hope this wasn't confusing, but appreciate your help.

This file will also end up being protected, so I need code that will this macro to run even when the sheet is protected.

Thanks
Brian

Thanks TMS,

Attached is a dumbed down sample of what I am trying to do.

Basically, I want everything the 'reset' if A4 changes - basically go back to a blank cell. If B4 changes, I want C4 & D4 to 'rese't, but A4 to remain as the current input.

C4 and D4 should be able to change without 'reseting' any thing; they are independent of each other and the values of these cells are determined based on the value of B4.

In my real application, I have a lot more of the C4/D4 cells but figure this sample gives the general idea.

Thanks again for the help!

Brian