Available choices:
(a). 10-12 hours..
(b). 8-9 hours..
(c). 7-8 hours..
(d). Less than 7 hours..
Q: Read the following article and answer the question. Article: Sleep is like food for the brain. Enough sleep helps the body and brain grow and develop. According to the National Sleep Foundation in the US, adults need seven to nine hours of sleep every night. For students aged 10 to 17, a healthy amount is about eight to nine hours per night. But many students don't get enough sleep because of early school start times and heavy homework. About 80 percent of middle school students don't get enough sleep, the China Youth and Children Research Center reported. For some students, they want to sleep early, but they keep worrying about their schoolwork and can't fall asleep quickly. For others, they may even have _ which might keep them awake all night. These sleeping problems greatly affect people's lives. Experts have shown that lack of sleep for teenagers leads to poor grades. Students without enough sleep cannot pay attention in class or do well in sports. Also, the lack of sleep may make them depressed . Luckily, with good sleep habits, it is easier to fall asleep. Some of these good habits include avoiding drinks that make people excited late in the day, going to sleep at the same time each night, and sleeping in a comfortable place that is dark, quiet, and neither too warm nor too cold. Question: If Li Hong is 15years old, how much sleep does she need every night? Answer:
A:
(b).