$1.$ In the Two$-$level Model of Multithreading Models if a thread makes a blocking system call what happen. $(3$ marks$)2.$ What is the benefit of using Thread Pools. $(3$ marks$)3.$ One of OS Services is Accounting "Explain in Arabic by your own language" $(2$ marks$)4.$ mention one vulnerability of the OS Simple Structure. $(2$ marks$)5.$ if possible give me $3$ examples each example with $3$ processes info; and the $3$ examples have the same average waiting time based on FCFS; if not possible explain, if possible give me examples and explain.