Does school start too early? Judging from the teenagers who yawn in class, it does. And those students aren't just lazy. Scientific studies have shown that school does start too early for young students. According to a series of studies done across the U.S., the later classes begin, the more academic performance improves. Boys and girls naturally stay up later and sleep in later. The trend begins around age 13 or 14 and peaks between 17 and 19.The teens also need more sleep in general, so forcing them to be up early for school cuts into their sleep time as well as their sleep rhythm, making them less ready to learn during those first-period classes. Practice is proving the science. Hundreds of school districts in the U.S. have experimented with later start times and the academic performance of students has improved as a result. A study of 9,000 high school students in three states showed that grades in science, math, English and social studies all rose when school began at 8:35 or later. What's more, attendance goes up and student depression goes down. The later school starts, the better the result, too. According to a study, delaying the opening bell from 7:30 a.m. to 8:30 a.m. paid off more than delaying only until 8:00 a.m. Students who get at least eight hours of sleep, and preferably nine, perform better than those who do not, so a greater delay raises the chances of achieving those numbers. So, it turns out morning class is just too early for students to learn chemistry equations or study an ancient poem. If you cannot change the school schedule, adjusting your own timetable to ensure eight hours' sleep can also help you perform better at school. The writer proves "School starts too early" by   _  .
doing survey doing experiments comparing statistics observing students' behaviour
Question: Does school start too early? Judging from the teenagers who yawn in class, it does. And those students aren't just lazy. Scientific studies have shown that school does start too early for young students. According to a series of studies done across the U.S., the later classes begin, the more academic performance improves. Boys and girls naturally stay up later and sleep in later. The trend begins around age 13 or 14 and peaks between 17 and 19.The teens also need more sleep in general, so forcing them to be up early for school cuts into their sleep time as well as their sleep rhythm, making them less ready to learn during those first-period classes. Practice is proving the science. Hundreds of school districts in the U.S. have experimented with later start times and the academic performance of students has improved as a result. A study of 9,000 high school students in three states showed that grades in science, math, English and social studies all rose when school began at 8:35 or later. What's more, attendance goes up and student depression goes down. The later school starts, the better the result, too. According to a study, delaying the opening bell from 7:30 a.m. to 8:30 a.m. paid off more than delaying only until 8:00 a.m. Students who get at least eight hours of sleep, and preferably nine, perform better than those who do not, so a greater delay raises the chances of achieving those numbers. So, it turns out morning class is just too early for students to learn chemistry equations or study an ancient poem. If you cannot change the school schedule, adjusting your own timetable to ensure eight hours' sleep can also help you perform better at school. The writer proves "School starts too early" by   _  .
A. doing survey
B. doing experiments
C. comparing statistics
D. observing students' behaviour
Answer: