question_answer

The value of \[S=\sum\limits_{k=1}^{6}{\left( \sin \frac{2\pi \,k}{7}-i\,\,\cos \frac{2\pi \,k}{7} \right)}\] is

A)  \[-\,1\]                         

B)  0

C)  \[-\,i\]                          

D)  i

E)  None of these