In which of the following combinations is buffer action expected?
$A$. $NH_4OH + NH_4Cl$
$B$. $HCl + NaCl$
$C$. $NH_4OH + HCl$ in $2 : 1$ mole ratio
Select the correct answer using the code given below:

The structure of the compound formed when nitrobenzene is reduced by lithium aluminium hydride $(LiAlH_4)$ is

One end of a spring of force constant $k$ is fixed to a vertical wall and the other to a block of mass $m$ resting on a smooth horizontal surface. There is another wall at a distance $x_0$ from the block. The spring is then compressed by $2x_0$ and released. The time taken to strike the wall is

The pole of the straight line $x+4y=4$ with respect to the ellipse $x^2+4y^2=4$ is

Observe the following reaction: $2 A + B \longrightarrow C$. The rate of formation of $C$ is $2.2 \times 10^{-3} \ mol \ L^{-1} \ min^{-1}$. What is the value of $-\frac{d[A]}{dt}$ (in $mol \ L^{-1} \ min^{-1}$)?

In a triangle,if $b=20, c=21$ and $\sin A=\frac{3}{5}$,then $a$ is equal to :