919 - Logical Reasoning

Question: Neighboring landholders: Air pollution from the giant aluminum refinery that has been built next to our land is killing our plants. Company spokesperson: The refinery is not to blame, since our study shows that the damage is due to insects and fungi.
Which of the following, if true, most seriously weakens the conclusion drawn by the company spokesperson?

1. The study did not measure the quantity of pollutants emitted into the surrounding air by the aluminum refinery.
2. The neighboring landholders have made no change in the way they take care of their plants.
3. Air pollution from the refinery has changed the chemical balance in the plants' environment, allowing the harmful insects and fungi to thrive.
4. Pollutants that are invisible and odorless are emitted into the surrounding air by the refinery.