Answer from cs61c-ei (Jing Chen 16669246) for Question 2
The cache index is essentially determined by (block address) % (# of cache blocks in the cache). Because the number of entries in the cache is a power of two, modulo can be computed by using only the low-order log2 (cache size of blocks) bits of the address. This makes for easy direct mapping between memory address and location in cache and easier computing since computers use binary.