Answer from cs61c-ei (Jing Chen 16669246) for Question 2
The ALUSrc controls whether the second ALU operand/input is 1) from second register file output (Read data 2) given that it is a R-type instruction and the mux is deasserted or 2) a sign-extended, lower 16 bits of the instruction given that it is a load instruction and mux is asserted.