During transcription,both strands of $DNA$ are not copied because:

• A
  If both strands act as a template,they would code for $RNA$ molecules with different sequences.
• B
  If both $RNA$ molecules are produced simultaneously,they would be complementary to each other and form double-stranded $RNA$.
• C
  The $RNA$ molecule would code for the same sequence.
• D
  Both $(a)$ and $(b)$ are correct.