The radius of a circle is increasing at the rate

Let $r$ be the radius and $C$ be the circumference of the circle at any time $t$. Then,

$C=2 \pi r$

$\Rightarrow \frac{d C}{d t}=2 \pi \frac{d r}{d t}$

$\Rightarrow \frac{d C}{d t}=2 \pi \times 0.7$          $\left[\because \frac{d r}{d t}=0.7 \mathrm{~cm} / \mathrm{sec}\right]$

$\Rightarrow \frac{d C}{d t}=1.4 \pi \mathrm{cm} / \mathrm{sec}$