Consider the following statements:

struct studentType$1$

$\{$

string name;

int ID;

double gpa;

$\}$;

studentType$1$ student$1,$ student$2$;

struct studentType$2$

$\{$

string name;

int ID;

double gpa;

$\}$;

studentType$2$ student$3,$ student$4$;

Which of the following statements is valid in C$++?$

a$.$ student$1.$ID $=$ student$3.$ID;

b$.$ student$2.$ID $=$ ID;

c$.$ student$1=$ student$4$;

d$.$ student$2=$ student$3$;