在$-\sqrt{7}$,$-\sqrt{3}$,$\sqrt{9}$,$0$这四个实数中，最大的是（　　）

    A warm drink of milk before bed has long been the best choice for those wanting a good night's sleep. But now a study has found it really does help people nod off（打瞌睡）—if it is milked from a cow at night.

    Researchers have discovered that “night milk” contains more melatonin(褪黑激素), which has been proven to help people feel sleepy and reduce anxiety（焦虑）.The study, by researchers from Seoul, South Korea, involved mice being fed with dried milk powder（奶粉） made from cows milked both during the day and at night. Those given night milk, which contained 10 times the amount of melatonin, were less active and less anxious than those fed with the milk collected during daytime, according to the study published in The Journal of Medicinal Food.

    Night milk quickened the start of sleep and caused the mice to sleep longer.

    While the effect of cow milk harvested at different time has not been tested on humans up to now, taking melatonin drugs has been suggested to those who have trouble falling asleep at night.

    Previous （以前的）studies have also showed that milk can be excellent for helping sleep because of the calcium content, which helps people to relax.

苹果内部腐烂时消耗一定量的葡萄糖可产生A mol的二氧化碳，其植物体叶片在正常生长时消耗同样数量的葡萄糖可产生二氧化碳（）

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Dear Wang Fang,

    Thank you for your letter. You wrote to ask me about American social customs(风俗) in their dinner party. You are asking the right person. I once lived in America for five years. Now I'll tell you something about it.

    It's a good idea to bring a small present when you go to a dinner party. Flowers are always nice, or you may bring a bottle of wine if you know that your friends drink it.

    You should arrive(到达) on time or five minutes late. Don't get there early. If you're going to be more than fifteen minutes late, you should call before.

    Try to be free at the dinner table. If you don't know about choosing(选择) the right fork or knife, just watch the other people, and follow them. If you still have no idea of what to do, ask the person beside you .

    If you like the food, say so. Of course, you'll thank them for the meal and for their kindness. It's also a good idea to send a card to thank them the day after.

Rose

Answer the following questions with No More Than 5 Words