This one is a little more complicated than just underlying a sentence.
Let's focus on the first two sentences here:
"Would it be right for the government to abandon efforts to determine at what levels to allow toxic substances in our food supply [AE]? Only if it can reasonably be argued that the only acceptable level of toxic substances in food is zero [RATSZ]."
We know "only if' introduces a necessary condition so we can diagram these two sentences as follows:
AE ===> RATSZ not RATSZ ==> not AE
The rest of the stimulus invokes the sufficient condition of our contrapositive, i.e. it cannot be reasonably argued that the only acceptable level of toxic substances in food is zero (not RATSZ).
This leads to the conclusion that it would not be right for the government to abandon efforts to determine at what level to allow toxic substances in our food supply (not AE).
Hope this helps! Please let us know if you have any other questions.
