325.The nation's population continues to rise at the rate of 12 million people per year.

英语四级译文:
该国的人口继续以每年1200万人的速度增长。

四级词汇讲解:
rate意为“速度;比例”，与rate有关的短语有:at this rate(以这样的速度)，birth rate(出生率)，divorce rate(离婚率)等。如:
The divorce rate is on the increase.离婚率正在上升。

本句中，at the rate of意为“以……的速度(增长等)”。如:
These new washing machines are being sold at the rate of 50 a day，这批新洗衣机以每天50台的销量售出。

英语四级考点归纳:
与at the rate of...形式类似的短语还有:

※ at the expense of意为“以……为代价;在损害……的前提下”。如:
1)I bought this shirt for you at the expense of 50 pounds and you dell me you do not like it?我花了50英镑给你买了这件衬衫，你却告诉我你不喜欢?
2)Nestle does not favor short-term profit at the expense of successful long-term business development.雀巢公司并不以损害长期的商业发展为代价去赢得短期利益。

※ at the cost of意为“以……为代价”。如:
At the cost of pain I regained the meaning of life.以痛苦为代价，我重新明白了生命的意义。