1. why is it difficult to evaluate thresholds for toxic pollutants? a. synergistic effects are difficult to account for. b. there is not enough research on the consequences of multiple exposures. c. lack of knowledge on the effect of possible interactions of mutiple toxins. d. all of the above

The options are all related, and the answer is D) All of the Above. 

The synergistic effects of exposure to multiple toxins are more difficult to quantify and study. As a result, there is not sufficient research on how much a system can take when exposed to multiple pollutants. This lack of research also means that we often do not even know what the effect of these interactions between multiple toxins are.

As a result of these gaps in our knowledge of the interactions between toxins, it is difficult to evaluate the threshold for pollutants, since a system will rarely feature only one pollutant in isolation.