Which of the following represents the repeating unit of a polyester?
−CO−CH2−CO−NH−CH2−CH2−NH−-\text{CO}-\text{CH}_2-\text{CO}-\text{NH}-\text{CH}_2-\text{CH}_2-\text{NH}-−CO−CH2−CO−NH−CH2−CH2−NH−
−CH2−CH(COOH)−-\text{CH}_2-\text{CH}(\text{COOH})-−CH2−CH(COOH)−
−CO−CH2−CO−O−CH2−CH2−O−-\text{CO}-\text{CH}_2-\text{CO}-\text{O}-\text{CH}_2-\text{CH}_2-\text{O}-−CO−CH2−CO−O−CH2−CH2−O−
−CH2−CH(OH)−-\text{CH}_2-\text{CH}(\text{OH})-−CH2−CH(OH)−
