What are some of the difficulties with measuring the volume of a gas? Select all that apply. (A) it could be soluble in liquid
(B) it can be difficult to contain.
(C) it varies depending on the container it is in
(D) it varies based on its mass

it is ---D because you can't measure gas and it's mass

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Research that involves a single subject is a single-subject research. It is also known as single case experiments.

Single-subject research is a group of research methods that are used extensively in the experimental analysis of behavior and applied behavior analysis with both human and non-human participants. 

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