中国大学MOOC: 阅读下列代码片段:class InterestTest ______ActionListener{……public void actionPerformed( ActionEvent event) {double interest = balance * rate/ 100;balance+ = interest;NumberFormat format= NumberFormat. getCurrencyInstance();System. out. printlb( balance = +Formatter. format ( balance));}private doublerate;}在下划线处应填入的正确选项是______。