Mapping: one-to-one and onto

Hello all!

I went through this and thought I had a handle on it till I spoke with a classmate of mine. Hopefully someone can set me straight.

a.) Is there a 1-to-1 function from the set {1,2,3} to the set {1,3}? Why or why not?

b.) Is there a function mapping {1,2,3} onto the set {1,3}? Why or why not?

c.) Is there a 1-to-1 function mapping the open interval (0,2) to the open interval (0,1)?

I answered no to (a) yes to (b) and no to (c)....my classmate crossed the sets in part (a) and has answered yes.....Why would you cross them?

Thanks!!!!!!!