Which of the following best illustrates "feedback" in development?

• A
  Tissue $X$ secretes $RNA$ which changes the development of tissue $Y$.
• B
  As tissue $X$ develops, it secretes enzymes that inhibit the development of tissue $Y$.
• C
  As tissue $X$ develops, it secretes something that induces tissue $Y$ to develop.
• D
  As tissue $X$ develops, it secretes something that slows down the growth of tissue $Y$.