Which one of the following statements is not correct?

The $\pi$-bonded organometallic compound which has ethene as one of its components is

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:

Number of complexes which will exhibit synergic bonding amongst $[Cr(CO)_6]$,$[Mn(CO)_5]$ and $[Mn_2(CO)_{10}]$ is ..... .

Which of the following carbonyls will have the strongest $C-O$ bond?

The number of bridged $CO$ ligands present in $Fe_2(CO)_9$ and $Co_2(CO)_8$,respectively are