at a rate of/at the rate of

两个用法都是正确的，而且基本意义相同。请考察下面的实例：

1. The train was now going at a rapid rate. (Davies) 火车那时正快速行驶。

2. The train was going at a rate of 80 kilometers an hour. (NAECD) 火车正以每小时八十公里的速度行驶。

3. at the rate of $20 an hour (CGEL) 以每小时20美元的价格(工资)

4. This money is taxed at the rate of $59\% \$ldots (EED) 这笔钱以 $59\%$ 的税率上税……请做下面的英语试题：

According to the new tax law, my money earned over that level is taxed at the ____ of 39%. A. ratio B. percentage C. proportion D. rate(全国英语专业四级,07-76)