Explain the idea of a hierarchy of memory. Why did the writers pursue it as a pyramid?

Please refer to this textbook: “A. Silberschatz, P. B. Galvin and G. Gagne, “Operating System Principles,”7th Edition, John Wiley & Sons Inc., 2006.”   And answer the following questions: Question:15 A system has the following blocks queued for writing: 8000, 3000, 5000, 2000. The most recently written block was 4500. The block read before that was 2500. (a) What sequence of writes will a C-SCAN algorithm generate? (b) What sequence of writes will a SCAN algorithm generate? (c) What sequence of writes will a FCFS algorithm generate? (d) What sequence of writes will a SSTF algorithm generate? (e) What sequence of writes will a LOOK algorithm generate?

What are the different garbage collection algorithms and how do they differ?

Provide an explanation of dynamic memory allocation using no more than a few sentences.