A、One third. B、Almost two thirds. C、More than two thirds. D、Almost half. B文中提到，根据调查，63％的美国成年人没有8个小时的睡眠时间，这8个小时是健康身体、安全优良工作表现所需要的

Americans are suffering from a serious sleep deficit as they spend more time on leisure activities. A world that "never goes to sleep" offers many diverse activities, but encourages unhealthy and sometimes anti-social lifestyles for American adults. Instead of working to live, they are living to work, a shift that has had a profound impact on their personal lives. Adults report spending less time sleeping, engaged in social and leisure activities. Most Americans say they suffer from sleep problems and when they go to sleep, many sleep alone even if they are married. These are some of the key findings in the National Sleep Foundation’s 2001 "Sleep in America" poll, which looks at the relationship between Americans’ lifestyles, sleep habits and sleep problems. Sleep deprivation continues to be widespread in America. According to the NSF poll, 63% of American adults do not get the recommended eight hours of sleep needed for good health, safety, and optimum performance. In fact, nearly one-third report sleeping less than seven hours each week night, though many adults say they try to sleep more on weekends. The poll shows good news and bad news. The good news is that many Americans say they don’t want to give up any more sleep in spite of their hectic lives. And they would sleep more if they were convinced it would contribute to their quality of life. The bad news is far too many adults still sacrifice sleep, which is unhealthy and counter-productive.
16. Why does the speaker say Americans are short of sleep?
17. What proportion of Americans does not get enough sleep according to the NSF poll?
18. What is the good news about the results of the poll?