The procedure below uses $3$ numbers stored in $$a0,$$$a1$ and $$a2.$

addi $sp,$sp$,-4$

sw sp$)$

add $t0,$a$0,$$a$1$

add $v0,$t$0,$$a$2$

$1$w sp$)$

addi $sp,$sp$,4$

jr $ra

$($a$)$ Write RTN comments by the side of each instruction describing its functionality.

$($b$)$ What is the code doing?

$($c$)$ Write a procedure Add$4$ that calls this one to add $4$ numbers.