Answer from cs61c-at (soemoe aung 16076861) for Question 2
The values come from previous instructions.
ID/EX:
sub $2, $1, $3
and $12, $2, $5

EX/MEM:
add $4, $3, $2
add $5, $4, $3

MEM/WB:
add $1, $1, $2
add $1, $1, $3
add $1, $1, $4