Which of the following formulas represents a secondary amide?
CH3CH2NHCH3\text{CH}_3\text{CH}_2\text{NHCH}_3CH3CH2NHCH3
CH3CONHCH2CH3\text{CH}_3\text{CONHCH}_2\text{CH}_3CH3CONHCH2CH3
CH3CH2CONH2\text{CH}_3\text{CH}_2\text{CONH}_2CH3CH2CONH2
CH3CON(CH3)2\text{CH}_3\text{CON(CH}_3)_2CH3CON(CH3)2