The author here is trying to answer whether it would "be right for the government to abandon efforts to determine at what levels to allow toxic substances in our food supply."
He posits "only if it can reasonably be argued that the only acceptable level of toxic substances in food is zero."
Only if introduces a necessary condition so this statement is diagrammed as follows:
AE ===> RATSZ
AE = abandon efforts RATSZ = reasonably argued only acceptable level toxic substances level is zero
The author then proceeds to show that the necessary condition does not exist, i.e. it cannot reasonably be argued that the only acceptable level of toxic substances in food is zero.
The contrapositive of the principle above is:
not RATSZ ==> not AE
By showing the necessary condition (i.e. RATSZ) does not exist, the author's main point or conclusion is that the sufficient condition (i.e. AE) does not exist.
Therefore, the main conclusion of the argument is that the government should not abandon efforts to determine at what levels to allow toxic substances in our food supply, or as stated in (A), "the government should continue trying to determine acceptable levels for toxic substances in our food supply."
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