If the rate of change of area of a circle is equal to the rate of change of its diameter, then its radius is equal to

$\frac{2}{\pi}$ unit $\frac{1}{\pi}$ unit $\frac{\pi}{2}$ units $\pi$ units

$\frac{1}{\pi}$ unit

area = $πr^2$ diameter = $2r$ so $\frac{d(πr^2)}{dt}=\frac{d(2r)}{dt}$ $2πr\frac{dr}{dt}=2\frac{dr}{dt}$ $⇒r=\frac{1}{π}$ unit