Answer the following questions about system call:$($a$)$ What is system call?$($b$)$ Give two examples in which your usef programs must use system call to perform some tasks? Describe the problems you would encounter if the system calls were not available and your programs had to carry out the tasks by themselves $($in terms of convenience and$/$or protection$).($c$)$ However, programmers usually don't directly initiate system calls. In this case, how do they access the system calls?