You list them in order, but if you were only interested in a certain one (say Navy), then you would only list Navy and that's all you would be considered for.
Sorry this is incorrect. You cannot only list one SA. You list them in rank order and hope you get the one you want. Also you must have a 3.0 GPA, a 222 on your PFE and an 80 on your MPA (military score). There are possibilities to get a waiver for everything except the PFE.
This info is straight from our CGA exchange students we sponsor. We do it every year and also have a DS currently at the CGA.