左图是____图形，它有____条对称轴．

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. "

在两伊战争中发生过举世震惊的“袭船战”，下列叙述符合史实的是

①“袭船战”指两伊战争中交战双方对向对方运输石油的油轮发动的袭击 ②“袭船战”影响到了非交战国的利益③美苏以此为契机加强了对海湾地区的控制  ④伊朗首先发动“袭船战”

说明光电效应产生的条件为：光频率大于临界值？