For the differential equation y'' + 4y = 0 , a general solution is of the form:
a) y = C₁ sin(2x) + C₂ cos(2x)
b) y = C₁ sin²(x) + C₂ cos²(x)
c) y = C₁ sin(x) + C₂ cos(x)
d) y = C₁ sin(4x) + C₂ cos(4x)