Answer from cs61c-af (Annie Tran 16811592) for Question 3
The purpose of the add unit is to compute the branch target as the sum of the incremented pc.
It is not possible to use a single alu to do the  functions of both the alu and adder of fig 5.10 because you would need to somehow keep state of the branch target then use the alu again to compare the the registers contents.  So, you would need to pass in a set of two differnt input to the alu at different time in the calulation which is not possible.