Answer from cs61c-bf (Wen Mei Lai 16790124) for Question 1
one is adding more than two variables at a time, such as a=b+c+d.  

one is accessing a element from a array, such as a=array[1].

the reason for it is MIPS execute one instruction per time.  each instructin contains three and exactly three registers. MIPs have limited registers, so variable such as array and struct are stored in memory instead of register. so the computer needs to to get it from memeory before using it.