关于声现象,下列说法正确的是()
A:声音在真空传播的速度最大
B:声源的振幅越大,音调越高
C:城市道路两侧的隔音墙是从噪声的产生环节控制噪声的
D:人能分辨出不同乐器发出的声音,主要根据它们发声的音色不同
材料一:见下图。
材料二:见下图。
如图,在平面直角坐标系中,直线y=2x+4与x轴、y轴分别交于A、B两点,以AB为边在第二象限作正方形ABCD,点D在双曲线y=kx上,将正方形ABCD沿x轴正方向平移a个单位长度后,点C恰好落在此双曲线上,则a的值是( )
Little Water Drop lives in the sea. It is raining. He wants to be ____
in the sky. The sun ____and he gets hot. Then he goes up to the sky. Now he is in a cloud. He flies ____rivers and mountains. He sees some thirsty ____. They need water. He falls down to the ____. A tree drinks him. Now he is inside the tree.