The amount present in a radio active element disintegrates at a rate proportional to its amount .The differential equation corresponding to the above statement is ($k$ is negative)

Question

$$\begin{array}{1 1}(1)\frac{dp}{dt}=\frac{k}{p}&(2)\frac{dp}{dt}=kt\\(3)\frac{dp}{dt}=kp&(4)\frac{dp}{dt}=-kt\end{array}$$