Benzene has alternativev double and single bond but its both length in carbon carbon moleules have same. Why?

Q. Cyclobutadiene (below) is like benzene in that it has alternating single and double bonds in a ring. However, its bonds are not same length, double bonds being shorter than the single bonds; the molecule is rectangular, not square. Explain why it would be incorrect to write resonance structures as shown.

Q. A cyclic hydrocarbon having carbon-carbon single bonds as well as carbon-carbon double bonds in its molecule is:
(a) C₆H₁₂
(b) C₆H₁₄
(c) C₆H₆
(d) C₆H₁₀

Q. Which of the following molecules has the shortest carbon-carbon single bond length?

Q. Cyclic hydrocarbon ${}^{\prime }{A}^{\prime }$ has all the carbon and hydrogen atoms in a single plane. All the carbon carbon bonds have the same length, less than $1.54A^{\circ }$, but more than $1.34A^{\circ }$. The bond angle will be

Q. Ordinarily the barrier to rotation about a carbon-carbon double bond is quite high but in compound P double bond between two rings was observed by NMR to have a rotational energy barrier of only about 20 cal/mol, showing that it has lot of single bond character.
The reason for this is