Question $2$: The four$-$bar mechanism shown below. The crank AB rotates at $90$ RPM clockwise.

Please answer the following: $($marks $=2+3+6)$

$($a$)$ Is this a mechanism or structure $($use equation to prove$)$

$($b$)$ How many instantaneous centers are in this mechanism.

$($c$)$ If the input Force $($at AB link$)$ is $50$ N and efficiency of the system is $69\%,$ find the output force $($piston $\text{'}$C$\text{'}).$