At Dallas/Fort Worth Airport, the lights are controlled by sensors that measure sunlight. They dim immediately when it's sunny and brighten when a passing cloud blocks the sun. A wall of windows at a University of Pennsylvania engineering building has built-in blinds   controlled by a computer program that follows the sun's path. Buildings are getting smarter and the next generation of building materials is expected to do even more. Windows could catch the sun's energy to heat water. Sensors that measure the carbon dioxide breathed out by people in a room could determine whether the air conditioning needs to be turned up. Many new materials and technology have been designed in the last 15 years. They are now being used in a wave of buildings designed to save as much energy as possible. They include old ideas, like "green roofs", where a belt of plants on a roof helps the building keep heat in winter and stay cool in summer, and new ideas, like special coating for windows that lets light in, but keeps heat out. As technologies such as sensors become cheaper, their uses spread. The elevators at Seven World Trade Center, which is under construction in New York, use a system that groups people traveling to nearby floors into the same elevator, thus saving elevator stops. People who work in the building will enter it by swiping   ID cards that will tell the elevators their floor, readouts will then tell them which elevator to use. The building also has windows with a coating that blocks heat while letting in light. More new building materials and technology are in development. A Philadelphia building firm is now working on "smart wrap" that uses tiny solar collectors to catch the sun's energy and transmitters   as wide as a human hair to move it. They are expected to change the face of the construction industry in the next ten years or so. The elevators at Seven World Trade Center are special because they can   _  .
send people to floors with fewer stops teach people how to use their ID cards make people stay very cool in summer help people go traveling in the building
Question: At Dallas/Fort Worth Airport, the lights are controlled by sensors that measure sunlight. They dim immediately when it's sunny and brighten when a passing cloud blocks the sun. A wall of windows at a University of Pennsylvania engineering building has built-in blinds   controlled by a computer program that follows the sun's path. Buildings are getting smarter and the next generation of building materials is expected to do even more. Windows could catch the sun's energy to heat water. Sensors that measure the carbon dioxide breathed out by people in a room could determine whether the air conditioning needs to be turned up. Many new materials and technology have been designed in the last 15 years. They are now being used in a wave of buildings designed to save as much energy as possible. They include old ideas, like "green roofs", where a belt of plants on a roof helps the building keep heat in winter and stay cool in summer, and new ideas, like special coating for windows that lets light in, but keeps heat out. As technologies such as sensors become cheaper, their uses spread. The elevators at Seven World Trade Center, which is under construction in New York, use a system that groups people traveling to nearby floors into the same elevator, thus saving elevator stops. People who work in the building will enter it by swiping   ID cards that will tell the elevators their floor, readouts will then tell them which elevator to use. The building also has windows with a coating that blocks heat while letting in light. More new building materials and technology are in development. A Philadelphia building firm is now working on "smart wrap" that uses tiny solar collectors to catch the sun's energy and transmitters   as wide as a human hair to move it. They are expected to change the face of the construction industry in the next ten years or so. The elevators at Seven World Trade Center are special because they can   _  .
A. send people to floors with fewer stops
B. teach people how to use their ID cards
C. make people stay very cool in summer
D. help people go traveling in the building
Answer: