Do you need complete silence to sleep well at night? While some people can sleep through a great deal of noise, others are woken up by the (9) sound. It appears that as we sleep, our brains produce rapid pulses, or waves, called sleep spindles. Research has (10) that our ability to sleep soundly may (11) on the number of sleep spindles we produce. Scientists conducted a three-day study on 12 volunteers, all of whom (12) themselves as good sleepers. On the first night, the scientists (13) the volunteers sleep in silence. On the following nights, however, they were subjected to various noises, ranging from the sound of a (14) tap to jet engine roars. Measurements of brain activity showed that the participants who reacted least to the noise were the ones who produced more sleep spindles, (15) on the quiet night. It is hoped that this discovery will (16) to new ways to help people who have difficulty sleeping. 9. A. lightest B. finest C. slightest D. weakest 10. A. exposed B. revealed C. displayed D. uncovered 11. A. depend B. involve C. rest D. base 12. A. assured B. declared C. claimed D. described 13. A. permitted B. agreed C. let D. allowed 14. A. pouring B. running C. streaming D. flowing 15. A. even B. precisely C. still D. exactly 16. A. head B. take C. guide D. lead