Auto choose drop-down value based on another drop-down value

Hello all

I am working with Nintex Forms and trying to get drop down field to change based on choices made in other drop-down fields. 

Example: I have drop-down A and B. Both have 3 items in.

I select choice 3 in drop-down A and then drop-down B changes to choice 2.

Then if possible, lock drop-down B from being edited as long as drop-down A contains choice 3.

The reason i am using drop-downs is that only one of the choices in drop-down should enforce the values further in the form.

I hope i have explained that clearly and appreciate any help that can be offered.

I am using Nintex Forms for SharePoint 2013. Not sure of the version number but It's the latest version downloaded Nintex (July 2017).

Thanks in advance