Prenell is filling his car with gasoline. Regular unleaded gas costs $1.89 per gallon. Which two equations show the proportional relationship between the total cost, c, and the price per gallon, p, of gasoline? (more than one answer)

a. p=1.89c
b. c=1.89/p
c. pc=1.89
d. c=1.89p
e. p/c=1.89
f. c/p=1.89