The equation of average power of a periodic signal x(t) is given as ___________

• A. ( sum_{k=0}^{ infty}|c_k|^2 )
• B. ( sum_{k=- infty}^{ infty}|c_k| )
• C. ( sum_{k=- infty}^0|c_k|^2 )
• D. ( sum_{k=- infty}^{ infty}|c_k|^2 )