Doctors already know that people who smoke can damage their hearing. The latest study in the journal Tobacco Control,【C1】______more than 3,000 US adults, suggests the same is true of passive smoking. Experts believe tobacco smoke may【C2】______blood flow in the small vessels of the ear. This could starve the organ of oxygen and lead to a build up of【C3】______waste, causing damage. The harm is different【C4】______that caused by noise exposure or simple ageing. In the study, the researchers from the University of Miami and Florida International University looked at the hearing test results of 3,307 non-smoking volunteers— some who were ex-smokers and some who had never smoked in their lifetime. The tests measured【C5】______of hearing over low, mid and high noise frequencies. To【C6】______passive smoke exposure, the volunteers had their blood checked for a byproduct of nicotine, called cotinine, which is made when the body comes into【C7】______tobacco smoke. This【C8】______that people exposed to second-hand smoke were far more likely to have poorer hearing than others, and to a degree where they might struggle to【C9】______a conversation in the【C10】______of background noise. Passive smoking increased their risk of hearing loss across all sound frequencies by about a third. Dr David Fabry, who led the research, said: "We【C11】______do not know exactly how much smoke you need to be exposed to in order to be at increased risk. 【C12】______we do know that the【C13】______for damage is very low. Really, the safe level of exposure is no exposure." Dr Ralph Holme, head of another research team, said: "We already knew from our own research that regular【C14】______smoking is a significant risk【C15】______leading to hearing loss and this new study is important as it【C16】______the increased risks posed by passive smoking too. Hearing loss can often be very【C17】______and lead to social isolation, if not quickly【C18】______Before you next【C19】______a cigarette, consider how it could impact not only【C20】______your own long-term hearing but your friends" and relatives" too."

【正确答案】 D

【答案解析】解析：由上句提到的不知道到底吸入多少烟会提高听力受损的风险，下句讲到真正的安全值就是零接触(no exposure)及本句表语is very low可知，本句讨论的是接触香烟的安全值。threshold通常指“门槛”，也有“阈值或临界值”之意，表示(物质辐射或浓度的)安全阈值，本句中可指造成听力损害的阈值很低。故选D项。