how to populate field value based on dropdown list ax 2012

Hi DAXers,

I'm new to X++

Thanks in advance for your support.

As shown in the picture below, I have created a new table includes 2 fields A & B used in a drop-down list in a master table form that includes other newly created 2 fields C & D. it is linked and drop-down list working properly. 

When I tried to modify C using the drop-down list that linked with the field A, I want to update field D by value from field B.

Thanks again,