For f(x)=arccos(x) defined on (−1,1), which statement about f′(x) is most accurate?
a) f′(x) is zero only at x=0 and negative elsewhere in (−1,1).
b) f′(x) changes sign exactly once in (−1,1) at x=21.
c) f′(x) is negative for all x in (−1,1).
d) f′(x) is positive for all x in (−1,1).