How does the Navy decide whether you get MM, EM, or ET when you go Nuke?

How do they decide whether you get MM, EM, or ET in the Navy's Nuclear Program? Do you get to put in any kind of request, is it based on ASVAB or NAPT scores or is it whatever they want you to be? I know its whatever the navy needs but do you have ANY say in the matter? PLEASE don't say its based on the needs of the navy I GET THAT.