A group of Russian criminals have stolen l.2 billion username and password combinations for more than 500 millione-mail addresses. It's the largest known theft on the Internet, according to a report of The New York Times.
Cyber security firm Hold Security discovered the security breach(漏洞 ).  The firm found that the group of criminals collected personal information from 420,000 websites, including household names and small Internet sites. The criminals were based in a small city in south central Russia. They hacked websites inside Russia as well as big companies in the U.S. and other countries ,The New Times reports.
The criminals found hundreds of thousands of weak websites and attacked their coding，Hold Security said. Alex Holden is chief information security officer of Hold Security. “The hackers did not just target U.S. companies；they targeted any website they could get，” Holden said. “And most of these sites are still easy to attack”.
According to Hold Security, the criminals have been using the stolen information to send junk mail through e-mail and on social networks like Twitter.  They can also use the 500 million stolen e-mail -addresses to plan other crimes.  They could use information from bank e-mails to steal your identity or sell the e-mail address to other criminals to make quick cash.
The reported break-ins are the latest events to raise doubts about security at big and small companies. Last winter, hackers stole 40 million credit card numbers and 70 million addresses, phone numbers and other personal information from the retailer Target Corp. The brand is still working to regain its shoppers' trust.
John Prisco is a CEO of a security firm. He says security hacks are more common than many people and companies realize. "This issue reminds me of an iceberg, where 90% of it is actually under water," Prisco said in an e-mailed statement. So many cyber breaches today are not actually reported, because companies are losing information and they are not even aware of it. "
Security experts believe hackers will continue breaking into computer networks unless companies become more protective of personal information.
What did a group of Russian criminals do according to the report?

A．They stole e-mail addresses of the Internet users.

B．They destroyed the websites of some big companies.

C．They attacked American companies in southern Russia.

D．They downloaded the secret information of some banks.

According to Alex Holden, the criminals entered some websites easily because          .

A．some websites don't have their own coding

B．most of the websites lack protective measures

C．Russia is more developed in computer science

D．the hackers are equipped with high technology

What does Paragraph 4 mainly talk about?

A．The criminals are fond of using social networks.

B．The criminals sell the stolen information for money.

C．The stolen information causes great economic losses.

D．The stolen information could bring harmful effects.

What does the underlined sentence in Paragraph 6 imply?

A．We cannot tell how large an iceberg is just from the part on the water.

B．Many companies are unwilling to report their information stolen.

C．Many companies are not aware their information has been stolen.

D．People usually can see just a small part of an iceberg.