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侧根一般是由母根的______细胞产生的，这种起源方式称为______。
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苔藓植物孢子体在一定时期可以进行一定的光合作用。( )
胚囊中助细胞壁的特点是______。
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听力原文： In an attempt to resist the rising cost of fuel and fight the pollution choking our cities, many outgoing students have taken to riding bicycles to school. The unfortunate result of this otherwise positive trend is the traffic problems caused by bicycles having to share the same road with cars. Unfortunately, traffic accidents involving bicycles are on the increase throughout the country, so this new solution has led to a dilemma. To solve this dilemma, special bike paths have been created on most roads leading to and from college campuses. It is essential that cyclists stay on those paths. It is necessary for the safety of those of you who ride bicycles as well as the motorists that you share the road with. Obviously, it may be inconvenient to ride in the bike paths at times, but we must insist on it. To enforce the necessity of staying on the paths, there are fines for bicyclists who ride down the middle of the street. The federal fines are currently running at $25 for a first-time violation, but if the law is continually violated, the fines will increase by different amounts depending on your local orders. The money that is collected from the fines will go towards a fund to help build better bike paths. You may have special bicycle bridges in your neighborhood right now. If not, you can count on seeing them in the future. We are committed to making bicycling a safe alternative to using cars and we appreciate your help in making that goal a reality. Questions 16 to 19 are based on the passage you have just heard. 16. What is the newest problem facing people who ride to school？ 17. What is the basis for the law that doesnt allow bicyclists in the middle of the street？ 18. How will the money from the fines be used？ 19. What can we learn about bicycles from the passage？16．
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