Susan Solomon Used an Observational Study Because

Susan Solomon used an observational study because:

A) the atmosphere is far too vast and complex to bring into the lab and manipulate in a controlled experimental study.
B) the use of human subjects is not legal in this country.
C) she didn't use an observational study, she used an experimental study.
D) her research was intended as a reconnaissance mission, and she later followed up with an experimental study.
E) there is no such thing as an experimental study; she used the only type of study that can be used: an observational study.