Answer from cs61c-eq (Nicholas Hwang 16732453) for Question 2
These are 1) rt from the regfile, 2) result of a memory read from the MEM/WB register and 3) result of an ALU operation from the EX/MEM register.

1) add  $2, $3, $1
2) lw   $2, 0($1)
   add  $3, $2, $1
3) add  $2, $3, $1
   sub  $3, $2, $1