Which of the following variable declarations are correct? If a variable declaration is not correct, give the reason$($s$)$ and provide the correct variable declaration. $(1,7)$

double conversion $=2\mathrm{.}5$;

$/$ I Ine $1$

char grade $=\text{'}B+\text{'}$ ;

$//$Line $2$

double $28\mathrm{.}5=$ num

$//$Line $3$

string message $=\text{'}\text{'}$First C$++$ course';

$//$Line $4$

int age $=18$ years;

$//$Line $5$

int perfectsquare;

$//$Line $6$

float $x,y,$ decimal;

$//$Line $7$