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。

youdao

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。

youdao

The 5th Corps' combat strength was estimated at 30, 000 troops when the U.S.-led invasion of Iraq began March 20, but it is unknown what toll heavy airstrikes and desertions have taken on its three infantry divisions and one mechanized infantry division, U.S. military sources told CNN.

CNN: Coalition makes key advances in northern Iraq

To an extent largely unknown to the American people and even to many U.S. policy-makers, the nuclear deterrent that has been the backbone of our defense posture for fifty years is becoming obsolete, unreliable and potentially ineffective.

CENTERFORSECURITYPOLICY: Towards a New Deterrent

The changes affect not only Delaware residents, but also any shareholder of a Delaware-incorporated company whose current address is unknown to the issuer or is a foreign address, including U.S. military personnel stationed abroad.

WSJ: State Governments Eye 'Inactive' Broker Accounts