Let a signal a1 sin (ω1 t + φ1) be applied to an astable LTI system. The corresponding output is a2 F sin (ω2 t + φ2). Then which of the following is correct?

• A. F is not necessarily a sine or cosine function but must be periodic with ω1=ω2
• B. F must be a sine or cosine function with a1 = a2
• C. F must be a sine function with ω1=ω2 and φ1 = φ2
• D. F must be a sine or cosine function with ω1=ω2