A、They look at students’ shoulders. B、They pay attention to the teacher’s explanation. C、They focus on the students’ reactions.

In the sunlit library at Jorge Prieto Elementary on Chicago’s northwest side, an experiment is under way. A provisional classroom has been set up. A whiteboard sits at the front of the room, and 20 eighth-graders are seated at the library tables. The math teacher Michael Hock is giving a lesson about the distributive property. Scattered throughout the room are some 30 other teachers. They aren’t wearing lab coats—but they might as well be.(19)They clutch clipboards and carefully monitor kids’ reactions to the teacher’s explanations, peering over students’ shoulders as they write answers. "What is the area of the garden?" Hock asks the students as he points to an illustration on the whiteboard. "Nestor, I haven’t heard from you today. " Nestor answers the question, and the 30 adults, including visiting teachers from Japan, scribble notes. (20)The exercise is called " lesson study." It’s a professional development strategy used extensively in Japan that essentially examines a teacher’s lesson and the way it’s delivered. Here’s how it works: (21)Teachers come up with a detailed lesson plan and explain ahead of time to colleagues the goals of the lesson. Then, one teacher tries the lesson out on a group of students, while dozens of other teachers watch what happens. Finally, the observers offer feedback and ideas for improvement. Though this kind of lesson study has been conducted in Japan for more than 100 years, (22)it is quite new in the United States. There’s been lots of talk about how Chicago should evaluate teachers. Lesson study is being billed as a way to help teachers improve.
19. What do some 30 other teachers do in class?
20. What is the "lesson study"?
21. How does lesson study work?
22. What can we know about lesson study from the passage?