Answer from cs61c-ep (Hao Wen Vivian Chong 15826235) for Question 2
The values come fron the write-back stage, the memory stage, and the values fetched in the instruction decode stage.

write-back stage ->
lw $s0, 0($t1)
add $s0, $s0, 1

memory stage ->
add $t0, $t0, 1
sub $t1, $t0, $t1

id stage ->
add $t0, $t1, $t2
sub $t3, $t4, $t5