回答下列问题:
Look! This is 1 apple tree. I can see many 2. The apples 3red. They are round. How are they? They are 4. Apples, apples, I love 5. An apple a day keeps the doctor away.
声音是由物体的____ 产生的,我们把正在发声的物体叫____ 。固体、液体、气体都可以作为声源。