Answer from cs61c-ee (Michael Shen 15963205) for Question 2
Feeding data from register file or ID/EX pipeline register:
  add $1, $2, $3
  sub $4, $5, $6

Feeding data from EX/MEM pipeline register:
  sub $2, $1, $3
  and $12, $2, $5

Feeding data from MEM/WB pipeline register:
  sub $2, $1, $3
  add $3, $6, $7
  or $13, $6, $2