Which of the following cannot be the Fourier series of a periodic sign

1.	Which of the following cannot be the Fourier series of a periodic signal?$
A.	x(t) = 2Cost + 4Cos7t
B.	x(t) = 2Cos‚âà√¨‚àö√ët + 4Cos7t
C.	x(t) = Cost + 0.7
D.	x(t) = 2Cost + 4Sin7t
Answer» D. x(t) = 2Cost + 4Sin7t

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