A spherical raindrop evaporates at a rate proportional to its surface area. The differential equation involving the rate of change of its radius $r$ with time $t$ is ... (where $k$ is a positive constant)
- A$\frac{dr}{dt} + k = 0$
- B$\frac{dr}{dt} - k = 0$
- C$\frac{dr}{dt} + kr = 0$
- D$\frac{dr}{dt} - kr = 0$
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