Which sentence from the article best supports the idea that the Barkley Marathons is a difficult race?
A.The racers rip out a particular page number of each book to prove they reached every required point.
B.Hundreds enter, but only about 40 are picked.
C.There is only one course map, so runners must copy the information to their own maps on race day.
D.Since then, only 15 people have successfully completed its estimated 100-mile distance.