Answer
9.8 m/s, often rounded to 10 m/s.
(The question asks for the "gain in speed" each second, so the answer has the units of a "gain in speed".)
Work Step by Step
The gain in speed per second is 9.8 m/s (often rounded to 10 m/s).
In other words, the acceleration is 10 m/s each second, or 10 $m/s^{2}$.
This is discussed on pages 46-47.
