A、They are very cheap and helpful. B、They will raise some privacy problems. C、They can help students keep fit. D、They show stude

　　A group of students in New York will soon be wearing controversial electronic monitors, which allow school officials to track their physical activity all the day in order to control the students’ obesity. The sports chairman for schools in Bay Shore, New York. ordered 10 Polar Active monitors, at $ 90 a piece, for use, starting this spring. The watch-like monitors count students’ heartbeats, find out their actions and even track their sleeping habits. (20) The information is shown on a color screen and gets passed to a password-protected website which students and teachers are all able to go into. The monitors are already in use in some schools in St. Louis and South Orange, N. J and have raised a lot of privacy concerns among some parents and observers. (21) But the Bay Shore sports chairman said, "It’s a very helpful way to fight the students’ obesity. It tells kids in real time, ’Am I active? Am I not active?’ We want to give kids the chance to become active. " In the South Orange-Maplewood School District, where earlier types of the monitors have been used for two years, upper-grade students’ marks in physical education classes are based in part on heart-rate monitors and activity sensors. Teachers use hand-held computers to collect information from each student’s monitor during class, then pass the information to the school computer system for storage and long-term tracking. (22) But privacy supporters and parents are still worried that schools are using such electronic monitors in other ways without families’ knowledge or agreement.
　　20. Where can teachers see their students’ physical activity?
　　21. What does the Bay Shore sports chairman think about the monitors?
　　22. Why do some parents worry about using the monitors?