The $...A...$ are the $DNA$ molecules that can carry a foreign $...B...$ segment into the host cell. Here $A$ and $B$ refer to:

• A
  $(A)-$ $Vector$,$(B)-$ $RNA$
• B
  $(A)-$ $Vector$,$(B)-$ $DNA$
• C
  $(A)-$ $Gene$,$(B)-$ $RNA$
• D
  $(A)-$ $Gene$,$(B)-$ $DNA$