下面程序生成CSV格式文件：王鑫,86,90,92,99,94,95 杨小虹,93,97,95,90,86,85 李静,89,98,99,94,90,85 程序如下：划线处的代码应该为？（ ）

a = [['王鑫', '86', '90', '92', '99', '94', '95'],
     ['杨小虹', '93', '97', '95', '90', '86', '85'],
     ['李静', '89', '98', '99', '94', '90', '85']]
f = open('成绩.CSV', 'w')
for i in a:
    f.write(','.join(i)+______)
f.close()