用所给动词的适当形式填空。 In many countries, 1_____(smoke) is becoming a bigger problem for young people. Most smokers 2_____  (start) in their teens (years of a person's age from 13 to 19 ) or even earlier. A study of 8,000 Beijing students last May 3_____(show) us that smoking was a problem for many Chinese kids. More than 21% of middle school students and 6% of primary school pupils(小学生) said they had smoked. 'If young people start 4_____(smoke) early, they will probably get addicted to nicotine (对尼古丁瘾).And it 5_____  (be) very hard to give up弃) later on,' said a professor. Every year, about 4 million(百万) people die (死亡) of smoking. And if people keep 6_____ (smoke), that number will go up to about 10 million a year by 2030, the World Health Organization (WHO, 世界卫生组织) says. So we have to 7_____(know) and tell others about the dangers(危险) of smoking. Everyone of us should 8_____(do) something useful to stop kids from 9_____(smoke), or one day we 10_____(see) what we won't see.