already offer such products，but many experts believe the market for the technology could explode due to electronic devices being developed for the Internet of Things．

"It's huge，"said Graham Martin，CEO of the EnOcean Alliance，a San Ramon-based group of businesses that promotes wireless energy-harvesting technologies．With the Internet of Things expected to combine billions of devices，"if they are all battery-powered，we'll have a problem because there's not enough lithium （锂） in the world，"he added．"So a lot of them will have to use energy harvesting．"

Among the most basic forms of the technology is body power．When certain materials are squeezed or stretched，the movement of their atoms creates an electrical charge．Automatic watches have employed the concept for decades，for example，by winding themselves when their user moves their arm．Now，the concept is being considered for a number of other devices．

In a contest seeking visionary ideas for wearable technologies，Intel awarded﹩5，000 for a concept to change the temperature difference between a person's body and a special piece of clothing they'd wear into electricity for mobile devices．Using sound to power devices is another energy-harvesting variation．Stanford University engineers are testing smart microchips （芯片） that create electricity from ultrasound （超声波） to power implantable （可植入的） devices that can analyze a person's nervous system or treat their diseases．

A textile （纺织品） research association in Spain is proposing to obtain electricity from radio waves that flow around everyone to power sensors sewn into clothes，which can monitor a person's heartbeat or other vital signs．

Research firm IDTechEx has estimated that annual global sales of energy-harvesting products could hit﹩2.6 billion by 2024，while WinterGreen Research predicts sales of﹩4.2 billion by 2019．

Obtaining stable energy from devices can be complex，however．For one thing，the motion that generates the electricity has to be constant to be useful．Moreover，the amount of power the devices produce depends on the person using them，according to a Columbia University study．It determined that taller people on average provide about 20percent more power than shorter ones when walking，running or cycling．

It's also unclear how eagerly consumers might welcome energy-harvesting products．While such devices are expected to cost less than battery-powered alternatives when compared over many years，experts say，people may continue buying ones with batteries merely because those would be cheaper in the short term．

1. Which"explode"in the following sentences has the most similar meaning to the word"explode"in Paragraph 2？ ______

A. They were clearing up when the second bomb exploded．

B. The continued tension could explode into more violence．

C. The population exploded to 40，000during the last tourist season．

D. The boss exploded when he heard of the resignation of the secretary．

What makes"energy harvesting"necessary according to the passage？ ______