premise: At the end of 2004, there were 25.7 mobile phones for every 100 Chinese, which exceeded the world average, statistics released at the 2005 China Mobile Telecommunications Terminal Development Forum showed.
  It also predicted that the number of mobile phone subscribers in China would top 380 million by the end of this year, and soar to 520 million by 2008 and 600 million by 2010.
  Incompatibility between different brands and even between different cellphones from the same manufacturer simply produces more waste.
  Suppose each subscriber buys a new cellphone every three years, and each cellphone is equipped with two batteries and one charger, an average 200 million batteries and 100 million chargers become wastes every year.
  "In fact, most people absent-mindedly put their outdated cellphones away -- after all, it's not easy to throw away something that used to cost dearly," said Sun Yuanming, a Chongqing-based sociologist. "But the proper disposal of these e-wastes remains a problem in the long run."
   What feature of smartphones makes people less likely to throw them away?
hypothesis: each cellphone is equipped with two batteries and one charger