For p(x)=x4−8x2+7, which of the following statements about its relative extrema is true?
a) p has relative maxima of equal value at x=−2 and x=2, and a relative minimum at x=0.
b) p has relative minima of equal value at x=−2 and x=2, and a relative maximum at x=0.
c) p has relative minima at x=−2 and x=2 but no relative maximum, because the derivative does not change sign at x=0.
d) p has relative minima of equal value at x=−2 and x=0, and a relative maximum at x=2.