A、It can help teachers interact with the students. B、It just shows the words to students. C、It can reduce the dust in the classr

A common challenge for many teachers is keeping their students focused in class. After all, many children can become distracted, bored or confused in a classroom setting, or abandon their concentration on a lesson plan. There are various ways teachers can encourage their students to stay focused and make learning more interesting. Based on a lack of students’ focus, teachers are sometimes given the impression that their lesson plan is not motivating or interesting to their students. However, the problem might simply be that the students need change and movement. It often helps for teachers to alternate quiet activities in class with more active and exciting lessons. Something changing the pace of your lessons and activities can make all the difference in helping your students stay focused, and ultimately enjoy the lesson you’ve prepared for them. Children tend to remain more focused when interactive teaching tools are involved. For instance, an interactive whiteboard in the classroom can help children feel included, and thus stay more focused. What’s more, it’s an effective teaching tool, helping instructors enhance lesson plans and students’ learning. So remember, the slightest change in how you deliver your lesson plans can make a world of difference when it comes to keeping your students focused. Above all， to reach the best effect in class, the first thing for teachers is to know the students’ ability to maintain attention—for instance, based on their age group— and enhance lessons accordingly for optimal focus and fun in the classroom.
Questions 19 to 21 are based on the passage you have just heard.
19. What should teachers do to make students enjoy the class?
20. What is the function of the whiteboard in the class?
21. What should teachers know first to have the best effect in class?