Answer
See explanations.
Work Step by Step
Based on the Intermediate Value Theorem, there must be a zero between $f(a)$ and $f(b)$. However, with $f(a)$ and $f(b)$ having the same sign, it may happen that there are an even number of zeros in between -- that is, the curve may cross the x-axis 2 times, 4 times, etc.
