I think the answer will be c
Answer:

Step-by-step explanation:

i don’t know for sure but i just figured it out....
Answer:
1. There are 8 big fish for every 10 small fish.
2. There are 4 big fish for every 5 small fish.
Now what you want to do is 21 + (-3). Since this is the same as saying 21 - 3, the answer is 18. Therefore, the answer is 18°F.