Which gas is present both in natural gas and biogas?

$(a)$ Complete the following equations:
$(i)$ $CH_4 + 2O_2 \longrightarrow CO_2 + 2H_2O + \text{Heat} + \text{Light}$
$(ii)$ $CH_3COOH + C_2H_5OH \xrightarrow{\text{Acid}} CH_3COOC_2H_5 + H_2O$
$(iii)$ $CH_3CH_2OH \xrightarrow[\text{443 K}]{\text{Conc. } H_2SO_4} CH_2=CH_2 + H_2O$

Which polymer is formed by reacting a compound containing two carboxylic acid groups and a compound containing two hydroxyl groups?

Which of the following is a natural polymer?

Which monomer is present in bakelite?

Which of the following is not a biopolymer?