The value of \({\left( {\frac{{ - 1 + {\rm{i}}\sqrt 3 }}{2}} \right)^{\rm{n}}} + {\left( {\frac{{ - 1 - {\rm{i}}\sqrt 3 }}{2}} \right)^{\rm{n}}}\) where n is not a multiple of 3 and \({\rm{i}} = \sqrt { - 1}\), is

• A. 1
• B. -1
• C. i
• D. -i