How to make a dropdown list cell default to blank, after another cell selection is made

Hi

I have no idea how to eve attempt this , so i hope someone can help.. I want the second dropdown list to default to BLANK, each time a new selection is made in the first dropdown list.. I.e i would like cell E306 to clear each time a selection change is made in cell D306.

I have supplied the file for your attention and thank you for any assistance you can provide..