Wang and Li turned to LED lights and created a LED door handle．When a person touches the handle，the UV light switches on，activating(使活跃)the titanium oxide on it and instantly killing 99．8 percent of the bacteria that might otherwise have been spread．

Julian Rios Cantu，Mexico

　　This inventor was just 13 years old when his mother was diagnosed with breast cancer．He watched in horror as the tumour(肿瘤)grew from the size of a green bean to the size of a golf ball.

　　After his mother recovered，Cantu set out to protect others from the disease．He developed wearable device that may be able to detect the early signs of breast cancer．If the project succeeds，it can save millions of lives．

21．Why did Cavé begin studying oil left on the ocean's surface?

A．To test her unique prediction．

B．To prevent the oil overflowing．

C．To apply for a patent on her invention．

D．To find the cause of baby dolphins'deaths．

22．What is special about the bacteria-killing door handle?

A．The titanium oxide is an activate chemical．

B．LED lights are used to provide power at night．

C．The UV light causes titanium oxide to kill bacteria．

D．The design has made it an affordable choice for most．

23．What can we infer about Cantu's wearable device?

A．It hasn't been put into use at present.

B．It can keep a record of tumors'growth．

C．It can stop the tumour from growing fast．

D．It helped to discover his mother's disease．

24．What can we learn from the teenagers mentioned in the text?

A．The problems of today are all solved by teenagers．

B．They are highly educated and truly creative．

C．A good beginning makes a good ending．

D．Necessity is the mother of invention．