Critical Reasoning

can someone tell me how the logic works!!
OA is marked by a box.
thank you.

Conclusion : Something other than Herbicide must be responsible for the population decrease.

Weaken... So we have say that "Herbicide is responsible for the population decrease."

1 says...

Use of Herbicide -- > Elimination of weeds --> reduced population of small insects --> hungry partridge --> decrease in partridge population