A function $f$ is **strictly increasing** on an interval if, on that interval:
- $f'(x) = 0$
- $f'(x) > 0$
- $f'(x) < 0$
- $f'(x) = \infty$
Show answer and 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 $f'(0) = 0$).