Related Rates and an Oil Slick

[Jacobpm64]

A ruptured oil tanker causes a circular oil slick on the surface of the ocean. When its radius is 150 meters, the radius of the slick is expanding by 0.1 meter/minute and its thickness is 0.02 meter. At that moment:

(a) How fast is the area of the slick expanding?
(b) The circular slick has the same thickness everywhere, and the volume of oil spilled remains fixed. How fast is the thickness of the slick decreasing?

Part B is easy, I just used a cylinder and figured it out. I'm sure I got it correct.

Part A is where I'm confused. When they say area, I wonder do they mean just the surface area of the top of the slick (a circle), or the surface area of the entire cylinder?

What do you people think?

 

[courtrigrad]

I think they mean just the surface area of the top of the slick.