Q$4.$ Answer the following questions about Proof of work alternatives.

$($a$)$ What are the advantages and disadvantages of POW?

$($b$)$ Name two cryptocurrency using POW, and two using POS.

$($c$)$ Consider the Pay and Play mechanism. What is the pay in POW, in POS, and in proof of capacity?

$($d$)$ Explain how the proof of elapsed time works. Why is it not suitable for cryptocurrency?