$6.$ Sequential Circuits Registers: Design a $4-$bit bi$-$directional register using a Dflipflop

$($a$)(2$ points$)$ Define the mode for shift right operation.

$($b$)(2$ points$)$ Define the mode for shift left operation.

$($c$)(2$ points$)$ What is the combinational circuit used to select the mode?

$($d$)(4$ points$)$ Implement the register