Rhodes drove to the meeting
Smith drove back from the meeting
Distance covered by both Rhodes and Smith = 140km

Target question=> What was the average speed, in kilometer per hour, at which Rhodes drove?
$$Average\ speed\ =\ \frac{dis\tan ce}{time\ taken}\ \ where\ dis\tan ce\ =\ 140km\ and\ time\ =??$$

Statement 1 => The average speed at which Smith drove was 70km/hr
The information provided was only about the average speed of Smith so it can only be used to estimate the time taken by Smith. Since the information is not related to Rhodes, it cannot answer the target question, Statement 1 is NOT SUFFICIENT

Statement 2 => Rhodes drove for exactly 2 hours
Time taken by Rhodes = 2 hrs
$$Average\ speed\ =\ \frac{dis\tan ce}{time\ taken}\ \ where\ dis\tan ce\ =\ 140km\ and\ time\ =\ 2hrs$$
$$Average\ speed\ =\ \frac{140km}{2hrs}\ $$
= 70km/hr
Statement 2 alone is SUFFICIENT

Answer = B