A 3-Gram Drop of Thick Red Paint Is Added to a Large

A 3-gram drop of thick red paint is added to a large can of white paint.A red disk forms and spreads outward, growing lighter at the edges.Since the amount of red paint stays constant through time, the density of the red paint in the disk must vary with time.Suppose that its density p in gm/cm² is of the form $p=k(t) f(r)$ for some functions k(t) of time and f(r) of the distance to the center of the disk.Let R(t) be the radius of the disk at time t.Which of the following equations expresses the fact that there are 3 grams of red paint in the disk?

A) $3=k(t)$$\int_{0}^{R(t) }$$f(r) 2 \pi r d r$
B) $R(t) =k(t) \int_{0}^{3} f(r) 2 \pi r d r$
C) $3=f(r)$$\int_{0}^{p(r) }$$k(t) 2 \pi r d r$
D) $p(r, t) =k(t)$$\int_{0}^{3} f(r) 2 \pi r d r$