A、The cardholders. B、The merchants. C、The banks. D、The police. C短文中提到，如果银行把所有责任都推给持卡人或商家，那么银行就没有动力去加强银行卡的安全性，因此，问题也永远无法得到妥善解决。由此

Cash or card? It’s a question we all face at the supermarket checkout. But new research from Europe suggests paying by card may not be so safe.(20)A weakness in payment terminal devices used to use bank cards is found.
    The way they’re attackable is very similar to how computers were attackable some 15-20 years ago.(21)So by sending unexpected network traffic to them, you can install viruses. Once that is the case then card cloning, the entire copying of payment cards including pin numbers could become a large scale problem.
    It took a few days to find this weakness. And it’s just a matter of time before criminals learn the same trick. Card fraud is already big business. 3 billion US dollars were lost to the crime in 2010, according to a report from Cambridge University. Many countries are now introducing new cards with a microchip inside. These use a more secure payment system.
    How to solve this problem? The most important thing from the point of view of policy is whether the banks are allowed to shift the burden of responsibility too much on to merchants and cardholders. In some countries either the merchant or cardholder is held responsible for fraudulent payment. In other words, banks have got off the hook. This is where the thing breaks down in some countries.(22)Because if the bank can move all the responsibility for fraud on to either the cardholder or the merchant, the bank doesn’t have an incentive to take care any more. If that’s the case, the worry is there’s less reason for banks to invest in better security. In that case, the problem can never be solved satisfactorily.
20. What is the main idea of the passage?
21. Why is paying by card unsafe?
22. Who should take the most responsibility according to the passage?