Transition elements form complex compounds due to:

Equilibrium constant for the reaction $H_2O_{(g)} + CO_{(g)} \rightleftharpoons H_{2(g)} + CO_{2(g)}$ is $81$. If the velocity constant of the forward reaction is $162 \ L \ mol^{-1} \ s^{-1}$,what is the velocity constant (in $L \ mol^{-1} \ s^{-1}$) for the backward reaction?

The rate constant for the reaction,$2 \ N_2O_{5(g)} \longrightarrow 2 \ N_2O_{4(g)} + O_{2(g)}$ is $4.98 \times 10^{-4} \ s^{-1}$. What is the order of reaction?

Match List-$I$ with List-$II$ and select the correct option.

List-$I$	List-$II$
$A$. Cardiac Arrest	$(i)$. Insufficient blood supply to the body
$B$. Heart Failure	$(ii)$. Heart muscle is suddenly damaged
$C$. Heart Attack	$(iii)$. Chest pain due to inadequate oxygen
$D$. Angina	$(iv)$. Heart stops beating

If $x > 0, y > 0, z > 0, xy + yz + zx < 1$ and if $\tan^{-1} x + \tan^{-1} y + \tan^{-1} z = \pi$,then $x + y + z$ equals to

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: Size of $Bk^{3+}$ ion is less than $Np^{3+}$ ion.
Reason $R$: The above is a consequence of the lanthanoid contraction.
In the light of the above statements,choose the correct answer from the options given below: