Answer from cs61c-aq (Anjali Koppal 16454392) for Question 2
The set of bits used is the lower log2(base 2) bits. the tag is the rest of the bits minus the offset. the cache needs to have a sequential set of words that can be indexed. if we use the top log2 bits we are not guaranteed that. therefore we use the lower log2 bits to index the cache and then compare the address with the tag to see if it is a valid address.