Here are the ranks and prizes.
D:(9:01 or more) Leech Hunter and Closet Key
C:(7:01 - 9:00) Leech Hunter and Closet Key
B:(5:01 - 7:00) Leech Hunter and Closet Key
A:(3:31 - 5:00) Submachine Gun, Leech Hunter, Closet Key
S:(3:30 or less) Rocket Launcher, Submachine Gun, Leech Hunter, Closet Key