Mike deposited ＄$$10000$$ in the bank. He earned an interest of＄$$2100$$ at the end of the second year. What is the interest rate per year of this bank? 
选项为:A:$21\textbackslash\%$  B:$11\textbackslash\%$  C:$10\textbackslash\%$  D:$9\textbackslash\%$  请回答其中一个选项.