一个按正弦规律变化的交变电流的图象如图所示，由图可知（   ）

①该交变电流的频率为0.2Hz；

②该交变电流的有效值为14.1A；

③该交变电流的瞬时值表达式为i=20sin0.02tA；

④t= $\frac{T}{8}$ 时刻，该交变电流大小与其有效值相等．

读下图，分析回答我国一些地区出现的环境问题的原因及治理措施。

电话是目前传递信息的最快方法。（  ）

在农田中试用某种农药防治害虫，使用年数越长，效果越差。对这一现象的下列解释正确的是（    ）

根据短文理解, 从A、B、C、D四个选项中选择最佳答案填空。

    Mabel is a beautiful girl. She likes all kinds of clothes, but she didn't1 her heart into her lessons when she was at school. With her friends' help, she finally finished middle school, 2 she learned little there. Of course, she can't do any important work and now she is a salesgirl in a 3 She has to work there and hopes to find a better one .

    One afternoon the girl argued with her mother angrily. So the following morning, . When she served an old man, she was very4 The old man chose a pair of shoes and bought them. Before he left, he asked, "How 5  will the shoes last?"

"It depends. If you don't use them .the shoes will  6  be broken. "

The old man  7 nothing and went away. That afternoon, while Mabel was  8  for supper, she cut her finger. She had to go to a hospital. That old man happened to be on duty. After Mabel got the medicine from him, she asked, "When will I get 9 ?"

    "It depends, "said the old man. "If you don't use 10 , you'll lose your finger. "