Yes, type "U" refers to Unknown for the Nominal Annual Rate.

是的，“U”型指的是未知的名义增长率。

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U - number of unknown answers in the first pass of the repetition, i.e. the number of questions that have one or more dots in their first column of the repetition scores field.

U -没答对问题的个数，也就是在重复学习评分栏里有一个或多个点的问题个数。

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And last year they saw how the rare hantavirus, once unknown in the U. s., emerged from mice to kill 30 people in as many as 20 states.

而去年，他们见识到罕见的病毒性流感，美国人过去完全陌生的病毒，如何从老鼠身上传出，在广达20个州境内夺走了30条人命。

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Earlier this year, a U. s. National Institutes of health study found that cellphone use can speed up brain activity, but it is unknown whether that has any dangerous health effects.

今年早些时，美国国家健康研究院发现，手机的使用可以加快大脑活动，但是这是否会造成健康危害尚未可知。

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Many drugs obtained from foreign sources that claim or appear to be the same as U. S. -approved drugs are, in fact, of unknown quality and may even be counterfeit.

许多来自国外药品看似与美国批准的药品一模一样，事实上，质量未知，甚至可能是假冒的。

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Also, a recent U. s. National Institutes of health study found that cellphone use can speed up brain activity, but it is unknown whether that has any dangerous health effects.

同时，美国国立卫生研究院在最近研究发现，使用手机会加快大脑的活动，此外还上不清楚有任何危害健康的影响。

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Also, a recent U. s. National Institutes of health study found that cellphone use can speed up brain activity, but it is unknown whether that has any dangerous health effects.

同时，美国国立卫生研究院在最近研究发现，使用手机会加快大脑的活动，此外还上不清楚有任何危害健康的影响。

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