下列（    ）对结构类型变量定义是错误的。

A:struct teacher  {int num;   int age;   }teach1;
B:struct {int num;   int age;   }teach1,teach2;
C:struct  {int num;   int age;   }teacher;   struct teacher teach1;
D:struct teacher {int num;   int age;   };   struct teacher teach1;