$($a$).$ What is the difference between common cause variation and special cause variation $($Q$15\mathrm{.}36$ Online Book Chapter $15)?(.5$ point$).$

$($b$)$ What is the difference between attribute control charts and variables control charts? $($Q$15\mathrm{.}40$ Online Book Chapter $15)?(\mathrm{.}5$ point$).$

$($c$).$ Why are $\stackrel{}{x}$ and $R$ charts used together $($Q$15\mathrm{.}41$ Online Book Chapter $15)?(\mathrm{.}5$ point$).$