题干

对于总数为N的一批零件,抽取一个容量为30的样本,若每个零件被抽到的可能性均为25%,则N=(  )

A:120

B:150

C:200

D:240

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答案(点此获取答案解析)

A

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阅读材料,回答问题。

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同类题3

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

    Nearly everyone has spent at least one night lying in bed wishing for sleep. But for many, it's a nightly struggle. The key, or secret, is to experiment. What works for some might not work as well for others.____

    Set a regular bedtime. Go to bed at the same time every night. ____Try not to break this routine on weekends when it may be tempting to stay up late. If you want to change your bedtime, help your body adjust.

    ____ If you're getting enough sleep, you should wake up on time naturally without an alarm. As with your bedtime, try to maintain your regular wake-up time even on weekends.

     Be smart about napping. While taking a nap can be a great way to recharge, especially for older adults, it can make insomnia(失眠)worse. ____ If you must nap, do it in the early afternoon, and limit it to thirty minutes.

    Fight after-dinner drowsiness(睡意). What should you do when getting sleepy before your bedtime? ____ For example, washing the dishes, or calling a friend. If you give in to the drowsiness, you may wake up later in the night and have trouble getting back to sleep.

A. Wake up at the same time every day.

B. Spend more time outside in the day time.

C. When it's time to sleep, make sure the room is dark.

D. Get off the couch and do something to avoid falling asleep.

E. If insomnia is a problem for you, consider giving up napping.

F. It's important to find the sleep strategies that work best for you.

G. Choose a time when you normally feel tired so that you can fall asleep easily.