Drop down list formula

Hi,

I am fighting to find the right solution for the following:

I have in Column A a drop down list with two variables.
-When they choose the first variable, they have to full-fill the Column B.
-When they choose the second variable, they don't need to full-fill the Column B.

I want when they choose the second variable to be filled the Column B with "N/A".

How can I do it?

Thank you in advance for your help.