Buzz Bunny is hopping up and down a set of stairs, one step at a time. In how many ways can Buzz Bunny start on the ground, make a sequence of hops, and end up back on the ground, without ever going below the ground level? (For example, one sequence of hops is up-up-down-down-up-down-up-down-up-down.) 
35
40
42
48
56
