A. if dic.count(user)==0: print('该学生不存在!') else: print(dic[user])

B. if user not in dic: print('该学生不存在!') else: print(dic[user])

C. if user in dic: print(dic[user]) else: print('该学生不存在!')

D. if dic.get(user,'error')=='error': print('该学生不存在!') else: print(dic[user])