Using carbo-loading to increase the store of glycogen seems to benefit only those athletes who engage in aerobic activities that are?

a. between 30 and 60 minutes
b. more than 90 minutes
c. less than 20 minutes
d. between 50 and 80 minutes.

i chose a for my first answer, but it told me i was wrong and i have looked all in the files and can't find it anywhere! would be greatly appreciated : )