Answer from cs61c-ev (Roland Carlos 16583196) for Question 2
We use that particular set of bits because they all map to certain location on the cache. We choose that set of bits because it fits with the equation for finding the mapping, which is (Block address)mod(Number of cache blocks in the cache).

We use the low-order bits to determine cache location because if we used the high-order bits, we would be basically mapping a bunch of bits that near to each other to a location (since we use the most significant bits). However, we handly spatial locality in a different way, so doing this way does not provide any speed bonuses.