What do the two short horizontal lines at the top of this bar graph indicate?

a. Average power usage decreased with carbohydrate supplementation only.
b. There is a significant difference in power usage from days 1-6 and between treatment types.
c. Average power use went down from days 1 to 6 for both protein and carbohydrate and carbohydrate only treatments.
d. Average power usage decreased with protein and carbohydrate supplementation only.