The expanding International Space Station has living space, a laboratory, storage modules

and solar power panels. Now it will get a backbone. The space shuttle Atlantis is set to take off for the station with a huge truss crammed into its cargo bay. The goal of the l l-day mission is to attach the central part of what will eventually become a support beam for the station.

The truss is made up of metal crossbars and filled with wiring, pipes, computers and other equipment. It is to be attached to the station during four spacewalks by the seven-member crew. The astronauts will also attach a mobile transport platform. to tracks atop the truss, the first part of a 300-foot (100-meter) mini-railroad that will carry a robotic arm and crew members back and forth across the station on future construction and repair jobs.

Atlantis's 25th flight will be the first in the 21-year history of the shuttle program for which the National Aeronautics and Space Administration will delay announcing the precise launching time for a civilian mission. Citing heightened security since Sept. 11, the agency said that liftoff would occur between 2 and 6 p.m. and that it would not announce the official time until 24 hours before the flight.

Before launching, NASA must determine that he 58-foot Canadian - built robot arm attached to the station is operating properly, since it will be critical in installing the truss. The multijointed Canadarm 2, as it is called, was installed last April and has since experienced periodic problems. In some operations recently, the brake on one of the arm' s seven joints has not released properly, causing it to malfunction.

To work around the problem, Canadian engineers wrote new computer software to operate the arm with six joints. The three-member resident crew at the station is testing the new operating procedures for installing the truss. If the software patch fails, the mission could be delayed several days until the problems are worked out.

Adding the center truss element begins a new phase for the ＄ 60 billion station, a 16-nation project led by the United States in cooperation with Russia, Japan, Canada and members of the European Space Agency. Over the next two years, six other girder sections will be added to serve as the home for four giant solar power arrays, heat radiators and other equipment that must be in place before attaching science modules from other nations.

The "truss" is______.

A．a space shuttle that carries a support beam

B．a mobile transport platform

C．a framework designed to support devices

D．a laboratory