In the eukaryotic cell cycle,cell fusion experiments show that:

• A
  When an $S$-phase cell is fused with a $G_1$-phase cell,the $G_1$-phase cell is stimulated to synthesize $DNA$.
• B
  When an $S$-phase cell is fused with a $G_2$-phase cell,the $G_2$-phase cell enters $S$-phase.
• C
  When a $G_1$-phase cell is fused with a $G_2$-phase cell,$DNA$ synthesis is induced in both.
• D
  When a $G_1$-phase cell is fused with an $M$-phase cell,the $G_1$ nucleus immediately enters $S$-phase.