Which of the following pairs of quantities vary directly?

a. the circumference of a cirlce and the diameter of the circle

b. the distance traveled in two hours and the average rate of travel

c.the number of gallons of gasoline bought and the cost of the purchase

d. the area of a circle and the radius of the circle

I think it is a,c and d. Is the answer correct? Does vary directly mean that it will always be a linear equation? Thanks to everyone for helping me and putting up with my random questions. I love doing word problems but I don't know much about algebra yet. so I am just trying to do the ones I can. Thanks. Vicky