Consider the following statements: struct studenttype1 { string name; int id; double gpa; }; studenttype1 student1, student2; struct studenttype2 { string name; int id; double gpa; }; studenttype2 student3, student4; which of the following statements is valid in c++? a. student1 = student4; b. student1.id = student3.id; c. student2 = student3; d. student2.id = id;