alleryA function $f$ is **strictly increasing** on an interval if, on that interval:
A$f'(x) = 0$
B$f'(x) > 0$
C$f'(x) < 0$
D$f'(x) = \infty$
Answer & Solution
Correct answer: B. $f'(x) > 0$
$f'(x) > 0$ throughout an open interval ⇒ strictly increasing there.
$f'(x) < 0$ ⇒ strictly decreasing.
$f'(x) = 0$ at isolated points OK (e.g., $x^3$ is strictly increasing even though $f'(0) = 0$).
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