18. They should either monitor their websites better so that children do not sign up too early, or they should adjust their websites to the needs of younger users.
他们要么更好地监控自己的网站，以免儿童过早注册，要么调整自己的网站，以适应年轻用户的需求。
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19. Researchers have found that students who sign up to tutor others work harder to understand the material, recall it more accurately and apply it more effectively.
研究人员发现，那些报名辅导他人的学生会更努力地去理解、更准确地回忆、更有效地运用材料。
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20. In fact, usually the system blocks anyone trying to sign up for a class they shouldn't be taking, who hasn't taken the courses you are required to do first as prerequisites.
事实上，这个系统通常会阻止任何想要注册他们不应该选的课程的人，也就是那些没有选必选先修课程的学生。
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21. Schools or teachers who sign up for the National Math Club receive a kit full of activities and resources, but there's no special teacher training and no competition attached.
22. The center's "Shop with Your Doc" program sends doctors to the grocery store to meet with any patients who sign up for the service, plus any other shoppers who happen to be around with questions.
23. From a political perspective, we need to understand the terms on which major countries like China and the USA might sign up to a global agreement, because at the moment we don't have that consensus.
