Seraphina is driving two hours to visit her family. For the first hour, she traveled at a speed of 56 miles per hour. Then, in the second hour, she traveled at a speed of 71 miles per hour. What is the percentage increase of Seraphina's speed? If necessary, round to the nearest tenth of a percent.

A. 21.1%
B.73.2%
C.26.8%
D.78.9%