Which of these methods has NOT yet been used to study the material of the interstellar medium?

Question

Quizplus · Accepted Answer

As of my last update, while methods such as scattering of starlight (A), emission of electromagnetic radiation by atoms and molecules (C), and absorption of light from more distant stars (D) have been used to study the interstellar medium, direct collection of dust and gas by spacecraft (B) has been limited and not extensively used for studying the material of the interstellar medium in the same capacity as the other methods.

Which of These Methods Has NOT Yet Been Used to Study