Choose ALL the valid C$++$ statements.

Group of answer choices

int a

cout $<<$ "Hello";

int b $="100"$;

double c $=10\mathrm{.}5$;

cout $<<$ Hello World $<<$ endl;

char d $=\text{'}$a$\text{'}$;

int main $=10$;

bool test;

return $0$;