Give the decreasing order of `C-H` bond length in ethane, ethene and ethyne and why?

Explanation

The decreasing `C-H` bond length is :
Ethane`(C_(2)H_(6))gt` Ethene `(C_(2)H_(4))gt` Ethyne `(C_(2)H_(2))`.
This is because the bybrid orbitals of C-atom involved in ovelapping with 1s orbital of H-atom are `sp^(3),sp^(2) and sp` respectively and their sizes are in the order. `sp^(3)gtsp^(2)gtsp`.
Alternatively: The `(C-C)` bond length gt `(C=C)` bond length `(C-=C)`bond length.