In order for a plant to initiate chemical responses to herbivory, ...

[xclash]

In order for a plant to initiate chemical responses to herbivory,
A) the plant must be directly attacked by an herbivore.
B) volatile "signal" compounds must be perceived.
C) gene-for-gene recognition must occur.
D) phytoalexins must be released.
E) it must be past a certain developmental age.

[clippers!]

I chose B and got the answer, hope that helps.