Which explains whether or not the function represents a direct variation?

a.) This function represents a direct variation because it passes through the origin and has a constant rate of change of $5 per hour.
b.) This function represents a direct variation because it has a positive, constant rate of change of $10 per hour.
c.) This function does not represent a direct variation because it does not represent the cost for 1 hour.
d.) This function does not represent a direct variation because the function rule for the cost is to add $10, not multiply by a constant.