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问题 选择题

如图所示是一交流电的电流随时间而变化的图象，此交流电的有效值是（   ）

A．4.5A

B．A

C．3A

D．5A

答案

答案：C

本题考查交流电有效值的计算问题；交变电流的有效值是描述交流电的一个重要物理量，在交流电不是正弦交流电时要用有效值的定义求解。（）解得：

阅读理解

根据书信内容判断下列句子的正误,正确的用"A,错误的用"B.

Dear Susan,

    My birthday is coming this Saturday. We're going to have a birthday party at home in the evening. Many of my friends are coming. We'd also like you to come. Could you come? Please tell me.

From:

    Linda King

   21 Chang'an Road

   Xi'an

                                                                                                                                        To:Susan Great

                                                                                                                                      32 Jiefang Road

                                                                                                                                       Xi'an

(    )1.Susan's   birthday   is on Saturday

( ) 2. This is a postcard from Susan to Linda.

( ) 3. Suan wants Linda to come to the party.

( )4. The birthday party is going to be held(举行) on Saturday morning.

( ) 5. Susan lives in Xi'an.
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单项选择题

1名12岁儿童由饮水含氟量0.3ppm地区迁居饮水含氟量1.2ppm地区，氟牙症发生的可能性为

A．0
B．25%
C．50%
D．75%
E．100%
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