Which of the following compounds does not react with $NaNO_2$ and $HCl$?

$A$ parallel plate capacitor with plate area $A$ and separation $d$ is filled with two dielectric materials as shown in the figure. The dielectric constants are $K_1$ and $K_2$ respectively. The capacitance will be

What does the part labeled '$A$' in the given figure represent?

If the harmonic mean of the roots of the equation $\sqrt{2}x^2 - bx + (8 - 2\sqrt{5}) = 0$ is $4$,then the value of $b$ is:

Which one of the following statements is correct about biodiversity?

The chemical reaction that involves the roasting process is