Which of the following cannot be the Fourier series expansion of a periodic signal?

• A. x1(t) = 2 cos + 3 cos 3
• B. x2(t) = 2 cos + 7 cos
• C. x3(t) = cos + 0.5
• D. x4(t) = 2 cos 1.5 + sin 3.5t