The number of bridging $CO$ ligand$(s)$ and $Co-Co$ bond$(s)$ in $Co_2(CO)_8$ respectively are

The correct statements about metal carbonyls are:
$A$. The metal-carbon bonds in metal carbonyls possess both $\sigma$ and $\pi$-character.
$B$. Due to synergic bonding interactions between metal and $CO$ ligand, the metal-carbon bond becomes weak.
$C$. The metal-carbon $\sigma$ bond is formed by the donation of a lone pair of electrons on the carbonyl carbon into a vacant orbital of metal.
$D$. The metal-carbon $\pi$ bond is formed by the donation of electrons from a filled $d$-orbital of metal into a vacant $\pi^*$ orbital of $CO$.
Choose the correct answer from the options given below:

Which of the following is an organometallic compound?

Assertion : $C-O$ bond in metal carbonyls is long.
Reason : There is delocalisation of electrons from filled $d$ orbitals into the empty $\pi^*$ orbitals on the $CO$ ligands.

$\pi$-bonding is not present in

Which of the following substances is used as an antiknock compound?