A computer has a L1-cache, L2-cache, main memory, and a disk used for virtual memory. If a referenced word is in the L1-Cache, 20 ns are required to access it L2-Cache, but not in the L1 cache, 40 ns are required to load it into the L1- cache and reference is started again Main memory but not in the cache, 60 ns are required to load it into the cache and reference is started again Hard disk but not in the main memory, 12 ms are required to fetch the word from the disk, followed by 60 ns to copy to the cache, and then the references starts again The L1-cache hit ratio is 0.9, L2-cache hit ratio is 0.8, and the main memory hit ratio is 0.6 s What is the average time in nanoseconds required to access a referenced word on this system