Identify the set of compounds that have the same functional group.
(a) C₂H₆, C₂H₄, C₂H₂
b) C₂H₂, CH₃COOH, CH₃CONH₂
c) CH₃CH₂CHO, H₂CO, CH₃CH₂CH₂CH₂CHO
d) CH₃NH₂, CH₃CH₂CONH₂, CH₃CH₂CONHCH₃
e) CH₃C(O)CH₃, CH₃COOH, CH₃CH₂CHO