A construction crew built 10 km of road in 40 days. Give 2 different unit rates using this relationship.
a) 0.25 km/day, 4 days/km
b) 0.4 km/day, 25 days/km
c) 0.5 km/day, 20 days/km
d) 0.3 km/day, 30 days/km