Which one of the following is not observed in biodiversity hotspots?

$A$ photon of energy $E$ ejects a photoelectron from a metal surface whose work function is $W_0$. If this electron enters into a uniform magnetic field of induction $B$ in a direction perpendicular to the field and describes a circular path of radius $r,$ then the radius $r$ is given by: (in the usual notation)

The figure shows a rough sketch of a meter bridge. The galvanometer $(G)$ shows zero deflection at a length $\ell \, cm$. Now,$R_1$ and $R_2$ are interchanged,then the balancing length increases by $25 \, cm$. Find the ratio $R_1/R_2$.

What is $X$ in the following reaction?
$MgCl_2 + 2H_2O \to X + 2HCl + H_2$

When alcohol vapor is passed over heated reduced copper $(Cu)$,the alcohol is converted into an alkene. Which type of alcohol is this?

If $g$ is the acceleration due to gravity on the earth's surface,the gain in the potential energy of an object of mass $m$ raised from the surface of the earth to a height equal to the radius $R$ of the earth is