当北半球大部分地区正午太阳升得最高，白昼时间最长，黑夜时间最短的一天是（）

    Today is Monday. It is sunny today. There is a sports meeting in our school. There are many students and players on the playground. Some basketball players are playing over there, and their classmates are watching. Some girls are running. There is a big tree on the playground. Some boys are jumping under the tree.

将塑料绳的一端扎紧，尽可能将其撕成更多的细丝，用干燥的手从上向下捋几下，观察到如图所示的现象．这是因为塑料丝带了____ 电荷（选填“同种”或“异种”）．