Answer:
D. Gas prcies just keep going up.
Step-by-step explanation:
The previous sentence, "with gas prices so high..." already stresses the price of gas, leading to the importance of walking or riding a bike. It goes on to talk about other benefits including fresh air and exercise. It is incorrect to go back onto gas prices; thus, this sentence is not only out of place, but it doesn't belong.
Hope this helps!