Final answer:
The original sentence should be changed to use the comparative form of 'safe' which is 'safer.'
Step-by-step explanation:
The correct answer is B. In the original sentence, the word 'safe' is used in the comparative form 'safer' to compare pesticides to insects. Since pesticides are being compared to insects, the correct form should be 'safer' instead of 'safe.' Therefore, the sentence should be changed to:
In the 1960s, when Silent Spring was published, many people believed that pesticides were safer than the insects they were meant to kill.
Learn more about Comparative form of adjectives