请帮我分析一句句子的语法结构,
问题描述：
请帮我分析一句句子的语法结构,
The chief reason for the population growth isnt so much a rise in birth rates as a fall in death rates as a result of improvements in medical care.
答
reason是主语; chief,for the population growth是修饰reason 的.isn't是谓语"so much a rise in birth rates as a fall in death rates as a result of improvements in medical care"其中a rise in birth rates as...