Which compound can form a polymer without needing another reagent?
H2NCH2CH2NH2\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2H2NCH2CH2NH2
HOOCCH2CH2COOH\text{HOOCCH}_2\text{CH}_2\text{COOH}HOOCCH2CH2COOH
H2NCH2CH2COCl\text{H}_2\text{NCH}_2\text{CH}_2\text{COCl}H2NCH2CH2COCl
CH3CH2CH2COCl\text{CH}_3\text{CH}_2\text{CH}_2\text{COCl}CH3CH2CH2COCl