Final answer:
When a plant disease kills all the fruit, it can have an impact on the birds in the ecosystem.
Step-by-step explanation:
The correct answer is B) Flock.
When a plant disease kills all the fruit, it can have an impact on the birds in the ecosystem. Without fruit, the birds may struggle to find food and their population may decrease. In this situation, the birds would form a flock, as they would gather together in a larger group to search for alternative food sources.