Read the following passage and choose the letter A, B, C, or D to indicate the answer to each of the following questions.

Carnegie Hall, the famous concert hall in New York City, has again undergone a restoration. While this is not the first, it is certainly the most extensive in the building’s history. As a result of this new restoration, Carnegie Hall once again has the quality of sound that it had when it was first built.

Carnegie Hall owes its existence to Andrew Carnegie, the wealthy owner of a steel company in the late 1800s. The hall was finished in 1891 and quickly gained a reputation as an excellent performing arts hall where accomplished musicians gained fame. Despite its reputation, however, the concert hall suffered from several detrimental renovations over the years. During the Great Depression, when fewer people could afford to attend performances, the directors sold part of the building to commercial businesses. As a result, a coffee shop was opened in one corner of the building, for which the builders replaced the brick and terra cotta walls with windowpanes. A renovation in 1946 seriously damaged the acoustical quality of the hall when the makers of the film Carnegie Hall cut a gaping hole in the dome of the ceiling to allow for lights and air vents. The hole was later covered with short curtains and a fake ceiling, but the hall never sounded the same  afterwards.

In 1960, the violinist Isaac Stern became involved in restoring the hall after a group of real estate developers unveiled plans to demolish Carnegie Hall and build a high-rise office building on the site. This threat spurred Stern to rally public support for Carnegie Hall and encourage the City of New York to buy the property. The movement was successful, and the concert hall is now owned by the city. In the current restoration, builders tested each new material for its sound qualities, and they replaced the hole in the ceiling with a dome. The builders also restored the outer walls to their original appearance and closed the coffee shop. Carnegie has never sounded better, and its prospects for the future have never looked more promising.

What was probably the most important aspect of the recent renovation?