$x^5$ is a constant quantity.

If $x$ varies as $y$, then which of the following is/are correct?

$1. x^2+y^2$ varies as $x^2-y^2$.

$2. \frac{x}{y^2}$ varies inversely as $y$.

$3. \sqrt[n]{x^{2}y}$ varies as $\sqrt[2n]{x^4{y}^2}$.

Select the correct answer using the code given below:

Check whether the following expression is with numbers only:

$2z-13$

The diameter of a circle is a line which joins two points on the circle and also passes through the centre of the circle. (In the below figure $\mathrm{AB}$ is a diameter of the circle; $\mathrm{C}$ is its centre). Express the diameter of the circle ($d$) in terms of its radius ($r$).