问题
单项选择题
Mary invested $8,400 for 6 months in a certificate of deposit paying 
percent simple annual interest. How much more interest would Mary have received if the interest rate on this certificate had been 9 
percent simple annual interest ？（）
A.$2.10
B.$21.00
C.$42.00
D.$210.00
E.$420.00
答案
参考答案：B
解析：
根据年单利息的计算公式可得
。
=4200×0.5%=21美元