Choosing Biology E or M

Hi! So I signed up last night to take the June Biology subject test. I want to take M, but there wasn’t an option on the College Board website to choose either, and my admission ticket just says “Biology E/M” or “E/M” (can’t remember which). Do you pick which one to take when you get there on the test day? Thanks!

You pick the one you want to take on test day.