题目：利用条件运算符的嵌套来完成此题：学习成绩>=90分的同学用

def main():

      s = int(raw_input('Enter a number:'))

  if s>=90:

        grade = 'A'

  elif s>=60:

        grade = 'B'

  else:

        grade = 'C'

        print grade,

if __name__ == '__main__':

    main()