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.

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.