From Boston to Los Angeles, from New York City to Chicago to Dallas, museums are either planning, building, or wrapping up wholesale expansion programs. These programs already have radically altered facades and floor plans or are expected to do so in the not-too- distant future.
In New York City alone, six major institutions have spread up and out into the air space and neighborhoods around them or are preparing to do so.
The reasons for this confluence of activity are complex, but one factor is a consideration everywhere — space. With collections expanding, with the needs and functions of museums changing, empty space has become a very precious commodity.
Probably nowhere in the country is this more true than at the Philadelphia Museum of Art, which has needed additional space for decades and which received its last significant facelift ten years ago. Because of the space crunch, the Art Museum has become increasingly cautious in considering acquisitions and donations of art, in some cases passing up opportunities to strengthen its collections.
Deaccessing — or selling off — works of art has taken on new importance because of the museum's space problems. And increasingly, curators have been forced to juggle gallery space, rotating one masterpiece into public view while another is sent to storage.
Despite the clear need for additional gallery and storage space, however, "the museum has no plan, no plan to break out of its envelope in the next fifteen years," according to Philadelphia Museum of Art's president.
This passage is mainly about the need for additional space in which of the following？

A neighborhood museum.
B. The Philadelphia Museum of Art.
C. Museums in the United States.
D. An aerospace museum.