What is known as 'Butter of Tin'?

The reason for the double helical structure of $DNA$ is the operation of

If the curves $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$ and $\frac{x^2}{25}+\frac{y^2}{16}=1$ cut each other orthogonally,then $a^2-b^2$ equals to

These are regarded as major causes of biodiversity loss :
$A.$ Over-exploitation
$B.$ Co-extinction
$C.$ Mutation
$D.$ Habitat loss and fragmentation
$E.$ Migration
Choose the correct option :

$\int \frac{1+x+\sqrt{x+x^2}}{\sqrt{x}+\sqrt{1+x}} d x$ is equal to

In a mutational event,when adenine is replaced by guanine,it is the case of