Let f be a function defined and continuous on the closed interval [a,b]. If f has a realtive maximum at c, and a I. f ' (c) exists
II. If f ' (c) exists then f ' (c)=0
III. If f ' (c) exists, then f '' (c) is less than or equal to 0
A: II Only
B: III Only
C: I and II only
D: I and III only