Weighted drop down

[ChaumeilDigital]

Hello,

In the Interfaces section, I'd like the value automatically selected in the first drop-down list to depend on the value selected in the second drop-down list.

Example:
Contents of list 1: A / B / C
Contents of list 2: X / Y / Z

If A is selected in the first drop-down list, select Z in the second drop-down list.

How do I do this?