Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.For a time, the Hubble telescope was the brunt of jokes and subject to the wrath of those who believed the U.S. government had spent too much money on space projects that served no valid purpose. The Hubble was sent into orbit with a satellite by the Space Shuttle Discovery in 1990 amid huge hype and expectation. Yet after it was in position, it simply did ...

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

For a time, the Hubble telescope was the brunt of jokes and subject to the wrath of those who believed the U.S. government had spent too much money on space projects that served no valid purpose. The Hubble was sent into orbit with a satellite by the Space Shuttle Discovery in 1990 amid huge hype and expectation. Yet after it was in position, it simply did not work, because the primary mirror was misshapen. It was not until 1993 that the crew of the Shuttle Endeavor arrived like roadside mechanics, opened the hatch that was installed for the purpose, and replaced the defective mirror with a good one.

Suddenly, all that had originally been expected came true. The Hubble telescope was indeed the “window on the universe,” as it had originally been dubbed. When you look deep into space, you are actually looking back through time, because even though light travels at 186,000 miles a second, it requires time to get from one place to another. In fact, it is said that in some cases, the Hubble telescope is looking back eleven billion years to see galaxies already forming. The distant galaxies are speeding away from Earth, some traveling at the speed of light.

Hubble has viewed exploding stars such as the Eta Carinae, which clearly displayed clouds of gas and dust billowing outward from its poles at 1.5 million miles an hour. Prior to Hubble, it was visible from traditional telescopes on earth, but its details were not ascertainable. But now, the evidence of the explosion is obvious. The star still burns five million times brighter than the sun and illuminates clouds from the inside.

Hubble has also provided a close look at black holes, which are described as cosmic drains. Gas and dust swirl around the drain and are slowly sucked in by the incredible gravity. It has also looked into an area that looked empty to the naked eye and, within a region the size of a grain of sand, located layer upon layer of galaxies, with each galaxy consisting of billions of stars.

The Hubble telescope was named after Edwin Hubble, a 1920s astronomer who developed a formula that expresses the proportional relationship of distances between clusters of galaxies and the speeds at which they travel. Astronomers use stars known as Cepheid variables to measure distances in space. These stars dim and brighten from time to time, and they are photographed over time and charted. All the discoveries made by Hubble have allowed astronomers to learn more about the formation of early galaxies.

According to the passage, a Cepheid variable is __________ .