For a Plant to Initiate Chemical Responses to Herbivory Before

For a plant to initiate chemical responses to herbivory before it is directly attacked by herbivores, which of the following must have occurred?

A) Volatile "signal" compounds must be perceived.
B) The hypersensitive response must be induced.
C) Vibrations must be sensed through the plant's leaves and stems.
D) A plant must have flowered at least once.
E) Systemin must be produced.