阅读下列材料，从每题所给的A、B、C、D四个选项中选出最佳选项。
Many people use the mobile phones as the alarm clocks and also use them to check e-mails before sleeping. It's a pity that these habits are becoming their sleeping problems.
Several years ago, my boss always played her mobile phone till very late and got badly ill because of that. Now she always leaves her phone in another room at night. And she asks her friends to do so. "I give each of my friends the same Christmas gift — an alarm clock. So they could stop using the phone as the alarm clock," she said.
If your mobile phone wakes you up in the morning, it may also keep you up at night. A study shows that people in mobile phone radiation mostly have sleeping problems. And according to a Swedish study, a quarter of young people in this country like having their phones around them. Most of them feel sorry for not answering messages. So they even have to answer messages to their friends before sleeping.
So what kind of people like to use mobile phones in their bedrooms better? A study has shown that three-quarters of people aged between 18 and 34 like to sleep with their phones near them better. That number becomes smaller in the middle age, but only people aged 65 and older like leaving the phone in another room when they are sleeping.
Ca(OH)2（aq），