辨认主题句：
Read the following paragraph and choose the best topic sentence from the four options that follow. Write the corresponding letter of your choice on the answer sheet.
Insect bites are not just an annoyance, but can come with serious consequences. People frequently use chemicals to protect themselves from insects, especially from mosquitoes (蚊子). However, these chemicals can have many unpleasant side effects, for both humans and the environment. Recently, there has been a lot of interest in non-chemical mosquito bite protection. A good candidate would be graphene (石墨烯)-a material made of carbon in very thin sheets that is already being used on clothes for various other applications. In a paper published in Proceedings of the National Academy of Sciences, researchers showed that graphene can provide a two-fold defense against mosquito bites. The ultra-thin yet strong material acts as a barrier that mosquitoes are unable to bite through. At the same time, experiments showed that graphene also blocks chemical signals mosquitoes use to sense that a blood meal is near, blunting their urge to bite in the first place.
【正确答案】：D
