Answer from cs61c-ee (Michael Shen 15963205) for Question 3
The hazard unit inserts bubbles by preventing the PC register and the IF/ID register from changing.  In addition, to stall the pipeline, we need to deassert all the control signals in the EX MEM and WB stages.  These control values are then percolated forward so that nothing is done during the clock cycle.
Fetching the next instruction prevents the hazard unit from inserting the same bubble over and over.