白磷在高压下隔绝空气加热后急速冷却，可得钢灰色固体﹣﹣黑磷，其转化过程如下：白磷 ​ 黑磷，黑磷比白磷稳定，结构与石墨相似．下列叙述正确的是（　　）

好的歌曲能够唱响时代的主旋律。《义勇军进行曲》、《黄河大合唱》谱出的最主要时代主题是（    ）

如图1所示，伍实同学用示波器、钢锯条和台钳研究声音的响度.他将钢锯条的下端夹紧在台钳上，上端用手扳动一下，使钢锯条振动发声。实验中，他进行了两次实验，第一次锯条发出的声音响，第二次锯条发出的声音轻，他同时观察到示波器上显示的波形幅度分别如图2、3所示，则他得出的实验结论是：____。

把下面的图片与所给的描述对号入座。
A        B          C 
D                E
____ He is of medium height and build, with two small eyes. He is a pop singer. And he likes playing basketball very much. He is very popular with young people.
____ He is short and of medium build. He has short black hair. He is a funny actor.
____ She is heavy. She has short brown hair. She always wears glasses. She sings very well.
____ He is very tall and has short black hair. He is a basketball player, and he plays it very well. Now he is a college student in Shanghai Jiaotong University.
____ She is thin, with long black hair and big eyes. She is a movie star. She is in the play "Palace", and she is very popular with young people.

美国独立战争开始的标志是（    ）