当发生下列哪种天气时，应及时关掉家用电器电源（　　）

It's raining. Ann meets Little Water Drop. She's talking with him.

Ann: Hi, where do you come from, Little Water Drop?

Little Water Drop: I come from the sea.

Ann: Oh，that's far from here.

Little Water Drop: Yes. I come here in a cloud. I fly over rivers and mountains.

Ann: Now you fall down here. How do you go back to the sky?

Little Water Drop: The sun shines and shines. I get hot. Then I go up to the sky. It's cool there. I am in a cloud.

Ann: Do you feel happy?

Little Water Drop: Yes, I do. I can help people. I am useful. People use water to wash clothes and vegetables, cook food, wash clothes, make drinks and so on. Farmers use water to grow crops and firemen use water to put out fires.

如图所示电路，开关S接到a后，小磁针静止时，A端是____ 极．将开关S由a换到b，调节变阻器的滑片P，保持电流表的示数不变，电磁铁的磁性____ （选填“增强”、“减弱”或“不变”）．

一位心理学家曾经做了这样一个实验。他在某中学新生开学的第一天，对其中的一个班宣布：你们是经过精选的、最有发展前途的学生。之后，他在十几年的追踪调查中惊人的发现，这个班的学生确实如他所说的，个个出类拔萃。这个事实说明了（  ）