For years, astronomers straggled to calculate the age of the universe. Estimates ranged from 10 to 20 billion years old, a frustratingly large spread. But that was before the launch of the Hubble Space Telescope, named after the astronomer whose discovery 70 years ago began the quest to learn the universe's age. Edwin Hubble found that the galaxies (星系) within the universe are speeding away from each other at a rate proportional to their distance. That expansion rate----the Hubble constant (常数)----is the key to calculating the age and size of the universe. But pinning down the constant requires precise measurements of the distances to far-flung galaxies. In May, a team led by Wendy Freeman of the Carnegie Institution, of Washington, D. C., announced the result of eight years of Hubble measurements: The universe is expanding at a rate of 21 kilometers per million light-years. That translates to an age of approximately 12 billion years for the universe. similar to the value for the oldest stare. "After all these years," says Freeman. "we are finally entering an era of precision cosmology (宇宙论). Now we can more reliably address the broader picture of the universe's origin, evolution, and destiny." Is the new number the last word Not quite. In June, astronomers using a large array of radio telescopes reported a measurement to a distant galaxy that would make the universe 15 percent younger than Hubble's results.

Passage 1What is the probable meaning of "pinning down" in the Paragraph 2（）

A．Forcing.
B．Discovering exact details about.
C．Pressing.
D．Knowing about.