Choose the mistakes．（找出下列每句中的错误).

(    )1. Our school is biger than your school.                 

                            A  B      C

(   )2. -Is Sam stronger than Lingling?        

           A           B      

         -Yes，he  isn't.   

                       C    

(    )3. Sam is  short than Daming.

                  A   B       C

(    )4.  The Changjiang River is longer than the yellow River. 

                  A                                B                 C

为"学生"表增加一个"平均成绩"字段的正确命令是ALTER TABLE 学生ADD【 】平均成绩N(5,2)。