The graph of f′(x) is a continuous piecewise-linear function that intersects the x-axis at x=−1 and x=4. It lies below the x-axis for x<−1, above the x-axis for −1<x<4, and below the x-axis for x>4. Which value of x is a location of a relative maximum of f?
a) x=4.5
b) x=−1
c) x=4
d) x=1.5