The ratio of number of oxygen atoms to bromine atoms in the product Q is _________ $$\times 10^{-1}$$.

The number of halobenzenes from the following that can be prepared by Sandmeyer's reaction is _________

The total number of hydrogen atoms in product A and product B is _________.

2-chlorobutane $$+\mathrm{Cl}_2 \rightarrow \mathrm{C}_4 \mathrm{H}_8 \mathrm{Cl}_2$$ (isomers)

Total number of optically active isomers shown by $$\mathrm{C}_4 \mathrm{H}_8 \mathrm{Cl}_2$$, obtained in the above reaction is _________.