Scientists from South Korea are trying to convert（转化）sound waves into electricity. The research could lead to charging a cell phone from a conversation or providing energy to the nation’s electricity system generated（产生）by the noise during rash-hour traffic.
“Just as speakers transform electrical signals into sound, the opposite process —turning sound into a source of electrical power — is possible,” said Young Jun Park and Sang-Woo Kim, the joint authors of a new article in the journal Advanced Materials.
Harvesting energy from phone calls and passing cars is based on materials known as piezoelectrics. When bent, piezoelectric materials turn that mechanical energy into electricity. Lots of materials are piezoelectric: cane sugar, quartz （石英）and even dried bone which could create an electrical charge when stressed. For decades, scientists have pumped electricity into piezoelectric materials for use in environmental sensors, speakers and other devices.
Over the past few years, however, scientists have made dramatic advances in getting electricity out of piezoelectric devices. In an experiment, by using sound waves, which at 100 decibels（分贝）were not quite as loud as a rock concert （a normal conversation is about 60-70 decibels）, the South Korean scientists produced a mild electrical current of about 50 millivolts（毫伏）. The average cell phone requires a few volts to function, several times the power this technology can currently produce.
“But the real question is whether there is enough surrounding noise to act as a power source as for a cell phone,” said McAlpine, a leading scientist. A consumer probably wouldn’t want to attend a rock concert or stand next to a passing train to charge his cell phone. The South Korean scientists agree but they expect to get a higher power output as they continue their work.
According to the two South Korean scientists, ________.

A．sound waves can travel faster than electricity

B．new materials can send cell phone signals better

C．using cell phones adds to heavy traffic in rush hour

D．electricity and sound can be transformed into each other

We know from the passage that piezoelectric materials ________.

A．can produce electricity when stressed

B．are good at changing electricity into sound

C．can reduce the noise of passing cars

D．have been widely used in phones and cars

It can be inferred from the South Korean scientists’ experiment that ________.

A．it is hard to change sound into electricity under current conditions

B．it is impossible to make use of loud sound

C．the technology has a long way to go to have a practical use

D．the technology can power cell phones easily

What McAlpine doubts about the technology is ________.

A．the noise pollution	B．the sound resource
C．the cost of piezoelectrics	D．the safety of devices