小陈编写了一段计算超市打折活动的程序，运行时报错，原因可能是下面哪项?

def discount(price,rate):

      final_ price = price *rate

old_price = float(input('请输入原价:”))

rate = float (input('请输入折扣率:"))

discount ( old price , rate )

print(“打折后价格是:',final price )