Correct answer is B.
The relation between height h, radius r and semi-vertical angle α is defined by
tan α = \(\frac{r}{h}\)
Given that h = 20cm
tan α = \(\frac{r}{20}-(1)\)
Given that α = 30° and \(\frac{dα}{dt}\) = 2°/sec, we have to find \(\frac{dr}{dt}\)
Differentiating (1) with respect to t, we get
Substituting values, we get