Cuprous ion is colourless,while cupric ion is coloured because

• A
  Both have unpaired electrons in $d$-orbital
• B
  Cuprous ion has a completed $d$-orbital and cupric ion has an incomplete $d$-orbital
• C
  Both have half-filled $p$ and $d$-orbitals
• D
  Cuprous ion has incomplete $d$-orbital and cupric ion has a completed $d$-orbital