people（人员）类派生出student（学生）类，添加属性：班号char classNo[7]；从people类派生出teacher（教师）类，添加属性：职务char principalship[11]、部门char department[21]。从student类中派生出graduate（研究生）类，添加属性：专业char subject[21]、导师teacher adviser；从graduate类和teacher类派生出 T