Yeah, basically which one you enjoy the least and/or are least good at. I was doing geography at the start of year 11, then switched to legal because I liked it better. However as a word of caution, legal is (at least for my class) very time consuming, often boring, and yes, as Gangles.McGee has said you have to read a lot.
Personally, I would get rid of bio. I hate any form of science.