let f be a twice differentiable real- valued function defined on the set of all real numbers greater than or equal to zero and less than or equal to one. suppose there is a unique x in the domain of f at which f'(x)=0.which of the following is true?
a. f attains the maximum at x.
b. if f"(x)>0 then f does not have a maximum value.
c. if f(x) is greater than f(0) and f(1) then f attains max at x.
d. f may not have a max value