challenge: What are the two main classification aspects of Operational Modal Analysis (OMA) methods?
  
  A. Linear or nonlinear, and time-invariant or time-varying
  B. Frequency domain or time domain, and deterministic or stochastic
  C. Parametric or non-parametric, and online or offline
  D. Frequency domain or time domain, and Bayesian or non-Bayesian
  Answer: 