Which of the following is a favourable factor for cation formation?

When a current of $2\,A$ flows in a battery from negative to positive terminal,the potential difference across it is $12\,V$. If a current of $3\,A$ flows in the opposite direction,the potential difference across the terminals of the battery is $15\,V$. The emf of the battery is ............... $V$.

If $x = \sqrt{\frac{2+\sqrt{3}}{2-\sqrt{3}}}$,then $x^2(x-4)^2$ is equal to:

$A$ neutrino is a particle,which is

The pelvic girdle of birds is attached to a complex structure formed by the fusion of the last thoracic,all lumbar,and the first five caudal vertebrae. This structure is called:

$\lim _{x \rightarrow 0} \frac{4^x-9^x}{x(4^x+9^x)}$ is equal to