Biodegradable polyesters, such as polylactic acid (PLA), are increasingly developed as sustainable alternatives to traditional plastics to combat global waste. Which functional group links the monomer units together in the backbone of these biodegradable polyesters?
−CONH−-\text{CONH}-−CONH−
−C=C−-\text{C}=\text{C}-−C=C−
−COO−-\text{COO}-−COO−
−O−-\text{O}-−O−