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Question:

Where should the following sentence be placed in the paragraph below? Insects that carry the disease can develop resistance to the chemicals, or insecticides, that are used to kill the mosquitoes. 1] Malaria, a disease spread by insects and parasites, has long proven to be difficult to treat. 2] Part of the explanation has to do with adaptation, or the ability of one generation to pass its strengths on to another. 3] Some insects are simply not affected by these insecticides. 4] Unfortunately, these are the insects that survive and go on to reproduce, creating another generation of insects that are immune to the current insecticides. 5] Many researchers have abandoned hope for insecticides as a cure for malaria, turning their attention instead to other forms of defense, such as protein-blockers that protect humans from the effects of the disease instead of from the carriers.

A After sentence 2
Explaination

The third sentence of this passage refers to "these insecticides," but there is no earlier reference to any insecticides in the paragraph. The sentence, "Insects that carry the disease can develop resistance to the chemicals, or insecticides, that are used to kill the mosquitoes," needs to be placed after sentence 2 for sentence 3 to make sense.