For strings r$,$s$,$v$,$w with r $$ s and v $$ w$,$ prove or disprove $($Prove $=$ explain in a few words why

it is true; Disprove $=$ give a counterexample$).$ Use shortlex order.

a$.$ rv $$ sw

b$.$ vR $$ w R

c$.$ w R $$ vR

d$.$ For any string x in Prefix$($v$),$ x v