CPU scheduling

$1.$ Consider three CPU$-$intensive processes, which require $10,20$ and $30$ time units and arrive at times $0,2$ and $6,$ respectively. How many context switches are needed if the operating system implements a shortest remaining time first scheduling algorithm? Do not count the context switches at time zero and at the end.

$($A$)1($B$)2($C$)$

Show all steps and workings with explanations so that it could be understood $.$

$($C$)3$

$($D$)4$