A:Because the tiger is friendly to them.
B:Because the tiger has no teeth.
C:Because the tiger has no mouth.
按课文《植物妈妈有办法》内容填空。
蒲公英传播种子要靠____;苍耳传播种子要靠____;豌豆传播种子要靠____。
关于机械波,下列说法中正确的是( )