Scientists have been taking a closer look at the lighting in our homes, offices and vehicl

es, and they're seeing a possible way to improve health.

Engineer E. Fred Schubert is talking about a new era of "smart" light sources. "We are looking at lighting systems that provide more than lighting," he says.

He's talking about light-emitting diodes(二极管),or LEDs. Most people know them as being quite small, like the lights that form. numbers on digital clocks. But recent technological advances have made them much more powerful, able to illuminate swimming pools and serve as traffic signals, for example.

Meanwhile, the lighting in offices and schools could be improved to help people stay healthy and productive, by acting on their internal body clocks. The 24-hour internal body clock is best known for governing cycles of alertness and sleep, and for producing jet lag(飞行时差) when people travel across time zones. Lights cues, especially blue light, help keep the clock on its daily cycle. We're pretty much blue-sky detectors. Our clocks count on bright days and dark nights.

But that's not necessarily what modern life delivers. During the winter in the northeast of America, for example, a person can commute roundtrip in the dark and sit all day in electric light that's fine for vision, but may be too dim to stimulate the body clock. That's called biological darkness.

How that affects people has been hard to document in the general population, but studies suggest such possibilities as seasonal depression, fatigue, sleep disturbances and maybe even cancer, especially breast cancer. Some studies suggest reduced productivity on the job. To counter that, architects and lighting engineers might someday take body clocks into account when they design lighting schemes. They may be encouraged to take steps like providing plenty of natural Night through windows and skylights, and installing bright blue LEDs near computer screens to give a dose of clock-adjusting light.

In the following places, where can we not find the LEDs？

A．Digital clock.

B．Traffic lights.

C．Swimming pools.

D．Cassette players' speakers.