Following are grades of nine students corresponding to the subjects Art, Math, CS and

Chem.

int$[][]$ grades $=\{\{85,78,95,71\},\{90,87,68,91\},\{99,82,73,80\},\{65,98,72,94\},\{88,$

$91,63,56\},\{79,92,69,83\},\{90,93,84,78\},\{67,90,83,88\},\{100,86,86,91\},\}$;

Write code to compute:

$1)$ The average CS grade of all students.

$2)$ The highest grade among all the CS grades.

$3)$ The number of students taking CS who have earned a B grade $($grades $80$ to $89).$

$4)$ The total number of B grades regardless of the subject $($grades $80$ to $89).$