$4.$ A system has four components, $\backslash (\backslash$mathrm$\{$A$\},\backslash$mathrm$\{$B$\},\backslash$mathrm$\{$C$\}\backslash )$ and D with individual reliability values $0\mathrm{.}985,\backslash (0\mathrm{.}890,0\mathrm{.}984\backslash )$ and $0\mathrm{.}997$ respectively. What is the system reliability if the components are arranged in:

a$)$ series $(5$ points$)$

b$)$ parallel $(5$ points$)$