Suppose a park has a total area of N km, and that A(t) km of this area is covered in weeds (with time t in days since the start of spring). If left untamed, the weeds will propagate at a rate proportional to the weeds-free area of the park. Which one of the following differential equations models this situation? dA dt = kA for some constant k > 0 dA = k(NA) for some constant k > 0 dt dA kA for some constant k > 0 dt N dA dt = k(A - N) for some constant k > 0