Dr. $F$. Went removed the coleoptile tips and placed them in agar for one hour. The agar block then caused curvature in decapitated coleoptiles. What does this experiment demonstrate?

• A
  It enables the identification and isolation of auxin.
• B
  It indicates the presence of a growth-promoting substance produced in very small quantities.
• C
  It supports the hypothesis that $IAA$ is the only auxin.
• D
  It demonstrates the polar movement of auxin.