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Light from distant quasars -- early galaxies that shine with tremendous brightness -- has given researchers a new clue to the origin of vast magnetic fields studding today's galaxies: They were running strong when the universe was only a third of its present age.

Astronomers had observed that radio emissions from quasars tend to be angled, or polarized, in such a way that powerful magnetic fields must have twisted them. The greater their distance from Earth, the more polarized their light. But researchers didn't know whether the magnetic fields were part of the quasar or were present in galaxies encountered by quasar light as it made its journey to our telescopes.

So a team led by astronomers from the Swiss Federal Institute of Technology (ETH Zurich) scanned more than 70 of those quasars using the European Southern Observatory's Very Large Telescope in Chile to look for signs of galaxies hiding in front of the quasars. Specifically, they checked for a feature called the magnesium(II) absorption line, a reduction in the intensity of light of a certain wavelength, which is a commonly used indicator that gas from a star-forming galaxy has soaked up that light.

The researchers report that light from quasars showing the magnesium(II) line was more strongly polarized than light from other quasars in the sample. The interpretation: that light did indeed pass through regular galaxies and that it likely acquired its polarization in the process.

They estimated the age of the magnetic galaxies by measuring the red shift of the absorption line—the observed reddening of light that occurs when galaxies move rapidly apart. The typical red shift of the inferred galaxies corresponded to an age of 5.2 billion years, study author Francesco Miniati says. Precision measurements of the cosmic microwave background peg today's universe at 13.7 billion years old.

Based on the passage, what can most reasonably be inferred about the relationship between two magnetic galaxies and their magnesium(II) absorption lines?

A.As the distance between the galaxies increases, the color of their absorption lines darkens.
B.As the galaxies age, their absorption lines become more pronounced.
C.As the galaxies move farther apart, their absorption lines become polarized.
D.As the brightness of the galaxies decreases, the intensity of the light from their absorption lines also decreases.

答案和解析

答案：A

解析：

Choice A is the best answer. The author states that the red shift of absorption lines is the observed reddening of light that occurs when galaxies move apart from each other. This suggests that the color of the lines becomes more deeply red, or darkens, as the distance between galaxies increases.