In the definition of the exponential function f left parenthesis x right parenthesis equals b superscript x baseline commaf(x)=bx, what are the​ stipulation(s) for the base​ b?
a. the base b must be greater than one.
b. the base b must be greater than zero and not equal to 1.
c. the base b cannot be a fraction.
d. the base b must be greater than or equal to zero.