Khorana and his colleagues synthesized an $RNA$ molecule with a repeating sequence of $U$ and $G$ nucleotides. The $RNA$ sequence "$UGU\ GUG\ UGU\ GUG$" produced a tetrapeptide with alternating chains of cysteine and valine. The codons for cysteine and valine are, respectively:

• A
  $UGG, GUU$
• B
  $UUG, GGU$
• C
  $UGU \text{ and } GUG$
• D
  $GUG \text{ and } UGU$