Which of the following oxidation states of chromium forms the most stable compounds?

Which one of the following ions participates in the production of $ATP$ from the oxidation of glucose?

What are the impurities present in ruby and sapphire?

Fill in the blanks:
$1.$ The copper layer on copper coins in the $UK$ is .......... .
$2.$ Copper-silver coins in the $UK$ are an alloy of ............ .
$3.$ The catalyst used for the oxidation of $SO_2$ to $SO_3$ is ......... .

Match the following:

List-$I$	List-$II$
$A$. $Co^{2+}$	$I$. Yellow colour
$B$. $Fe^{2+}$	$II$. Dark green colour
$C$. $Ni^{2+}$	$III$. Blue colour
$D$. $Cu^{2+}$	$IV$. Pale green colour
	$V$. Pink colour

The correct answer is

$2KMnO_4 \xrightarrow{200^{\circ}C} K_2MnO_4 + MnO_2 + O_2$. Here,'$X$' is $K_2MnO_4$. Which of the following statements is correct for '$X$'?