Which is not true about metal carbonyls?

• A

  Here CO acts as a Lewis base as well as l,ewis acid

• B

  Here metal acts as Lewis base as well as Lewis acid

• C

  Here $\mathrm{d\pi }-\mathrm{p\pi }$ back bonding takes place

• D

  Here $\mathrm{p\pi }-\mathrm{p\pi }$ back bonding takes place