Decreasing order of C-C bond length is
1.Ethane
2.Ethene
3.Ethyne
4.Benzene
4>3>2>1
1>4>2>3
2>1>4>3
4>1>3>2
Ethane-1.54. Ethene-1.33 , Ethyne-1.2, Benzene-1.39 Armstrong units