Consider the following string of memory references in terms of the page number requested:

$2,7,1,3,5,3,4,3,7,6,1,7,5,0,5,4$

How many page faults would occur for the following replacement algorithms, assuming four frames which are all initially free? $(6$ points$)$

a$)$ FIFO

b$)$ LRU

c$)$ OPT