$\sum_{n=1}^{20}\left[\sin \left(\frac{2 n \pi}{21}\right)-i \cos \left(\frac{2 n \pi}{21}\right)\right]=$

• A

  $1$
• B

  $-1$
• Correct answer
  C

  $i$
• D

  $-i$

Hence, the correct answer is option: 'C'.