话筒的作用是：当人对着话筒说话时，声波使话筒里振动膜做相应振动，使金属盒里的　____ 　发生忽松忽紧地挤压变化，它的　____ 　也就随之发生忽大忽小的变化，这时电路里就产生了强弱按声音振动而变化的　____ 　．

    Mr. White lives in the countryside with his wife, Mrs. White. From Monday to Friday he works in a small office near his house. He is free on Saturdays and Sundays. He has a nice garden near his house. He likes growing flowers and he usually works in the garden on Saturdays and Sundays. The flowers are colourful and beautiful. Mrs. White likes them very much. She often helps Mr. White. They live happily together.

图甲、图乙表示城市人口密度和城区在15年间的变化。读图回答以下题：