The authors propose all of the following as possibly contributing to the "missing matter" in spiral galaxies EXCEPT

It can be inferred from information presented in the passage that if the density of the universe were equivalent to significantly less than three hydrogen atoms per cubic meter, which of the following would be true as a consequence?

The authors' suggestion that "as much as 90 percent of the mass of the universe is not radiating at any wavelength with enough intensity to be detected on the Earth" (in the highlighted text) would be most weakened if which of the following were discovered to be true?

The authors' study indicates that, in comparison with the outermost regions of a typical spiral galaxy, the region just outside the nucleus can be characterized as having

The passage is primarily concerned with

The passage suggests which of the following about the identification of a species as a keystone species?

Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage (in the highlighted text)?

Which of the following hypothetical experiments most clearly exemplifies the method of identifying species' roles that the author considers problematic?

The passage mentions which of the following as a factor that affects the role of P. ochraceus as a keystone species within different habitats?

Which of the following best describes the function of the sentence in the highlighted text?