Populate value in a text attribute from two different fields through a condition

Summary

Populate value in a text attribute from two different fields through a condition

Content

Hi,

I have 3 attributes in CPQ system;

Attribute "A" in transaction line with type "single select menu". 
Attribute "B" in transaction line with type "Text". 
Attribute "C" in configurations with type "single select menu".

Now, I want to write a condition on a modify action button that;

If there is a value  selected in Attribute "A" and no value selected in Attribute "C" then value  from Attribute "A" gets populates to Attribute "B"

If there is a value  selected in Attribute "C" and no value selected in Attribute "A" then value  from Attribute "C" gets populates to Attribute "B"