【单选题】
下列（）对结构类型变量定义是错误的。
①
structteacher{intnum;intage;}teach1;
②
struct{intnum;intage;}teach1,teach2;
③
struct{intnum;intage;}teacher;structteacherteach1;
④
structteacher{intnum;intage;};structteacherteach1;