Failing Hückel

Chemistry Level 2

Cyclodecapentaene or [10]annulene is a compound which is unstable, despite its resemblance to benzene, an aromatic compound. What criterion of Hückel's rule does cyclodecapentaene fail to meet?

It is not cyclic.
It is not planar.
It does not have $4n+2$ $\pi$ electrons in its delocalised, conjugated $p$ -orbital cloud.
Not all its atoms participate in delocalising electrons.