Why is the cuprous ion colorless while the cupric ion is colored?

• A
  Both have unpaired electrons in their $d$-orbitals.
• B
  The cuprous ion has a completely filled $d$-orbital,while the cupric ion has an incompletely filled $d$-orbital.
• C
  Both have half-filled $p$ and $d$-orbitals.
• D
  The cuprous ion has a half-filled $d$-orbital,while the cupric ion has a completely filled $d$-orbital.