Assertion (A): In but-1-en-3-yne, $\text{CH}_2=\text{CH}-\text{C}\equiv\text{CH}$, the single bond that joins C-2 to C-3 is shorter than the carbon-carbon single bond of ethane, $\text{CH}_3-\text{CH}_3$. Reason (R): An $sp$ hybrid orbital has $50\%$ s character and an $sp^2$ orbital about $33\%$, both more than the $25\%$ of an $sp^3$ orbital, so their electrons are held closer to the nucleus.