I would go with c because its tells you right in there that the ecosystem help who lives around that area.
Through my research I found matching questions with similar answers:
A) macrophages
B) antigens
C) antibodies
D) lymph
The best answer to fill in the blank would be the third option C) Antibodies which in the name anti means against, so since the word anti is the same as against, its a battle between antibodies and invaders, so that's why its C).
Fruit flies share 75% of the genes that cause diseases with humans, so scientists can learn about human genetics by studying fruit fly genetics.
Your answer is going to be epididymis
