America's greatest technological achievement, the Hoover Dam, now has a companion piece, a bridge held up by the longest arch in the Western Hemisphere . The Mike O'Callaghan-Pat Tillman Memorial Bridge, which opened this month and connects the U.S. states of Arizona and Nevada, crosses the vast chasm 890 feet above the Colorado River that is controlled by the dam. The striking 1,900-foot-long structure will improve traffic in the region and help protect the dam from being destroyed. It is the seventh highest bridge in the world, behind four in China, one in Papua New Guinea and one in the US state of Colorado. "The Hoover Dam is the greatest civil engineering achievement in America's history," said the bridge project manager Dave Zanatell. "Our goal was not to outdo or outshine it. Our goal was to, in a respectful way, do something that would be great for our generation and that would stand besides Hoover Dam in a respectful and quality way that would become a part of Hoover's legacy ." Just as the Hoover Dam was built in the heart of the Great Depression and was seen as an example of the nation's can-do spirit, some hope this project can also provide some _ . The bridge is surprising: at 1,050 feet, its support arch holds up a roadway that lies on 300-foot-long concrete pillars , some of the tallest in the world. It contains 16 million pounds of steel. The idea of the bridge came into being in the 1960s because the top of the Hoover Dam has been a narrow two-lane road that is the fastest route from Arizona to Las Vegas and then the Pacific Northwest and Canada. Access to the dam from each direction is a dangerously winding route, but massive trucks and passenger vehicles shared it for decades. During the day, when thousands of tourists travel to the dam from Las Vegas three times as many accidents as on a normal road will happen. The bridge has a sidewalk on the side facing the dam. The wall on both sides is 54 inches high, so walkers can take photos there. The text is mainly about _ . A) the Hoover Dam B) American western development C) a tour along the Colorado River D) a recently built bridge
A:
D