题目内容：
根据下列文章,请回答题。Credit Card Only Works When Spoken ToAcredit card that will not work unless it hears its owner's voice could become an important weapon in the fight against fraud (欺骗).The card requires users to give a spoken password that it recognizes using a built-in voice-recognition chip. The idea is to prevent thieves using a stolen card or fraudsters using someone else's credit card details to buy goods online.
Amodel built by engineers at Beepcard in Santa Monica, California, represents the first attempt to pack a microphone, a loudspeaker, a battery and a voice-recognition chip into a standard-sized credit card.
They are not quite there yet: the card is the length and width of an ordinary credit card, but it is still about three times as thick. The company now plans to make it thinner.
The voice card is based on an earlier Beepcard technology designed to prevent fraud in online transactions. This earlier card has no microphone, but has a built-in loudspeaker that it uses to "squawk" (发出叫声) a voice ID signal via a computer's microphone to an online server.
By verifying (证实) that the signal matches the card details, the server can establish that the user is not simply keying in a credit card number but actually has the card to hand. The ID code changes each time the card is used in a pre-ordered sequence that only the server knows.
This prevents fraudsters recording the beeps, noting the card details and then playing back the audible ID when they key in the details later. But this earlier technology cannot prevent fraudulent use of stolen cards. The new one can.
The new voice card also identifies itself by its ID squawk, but it will not do this until it has verified the legitimate (合法的) user's spoken password. Thieves will be unable to use the card because even if they knew the password they would have to be able to copy the owner's voice with a high degree of accuracy.
The challenge for Beepcard has been to develop voice-recognition and audio circuitry that can be powered by a mini battery embedded (嵌入的) in a credit card. TO maximize battery life, the electronics are only switched on when the card is being used. Pressing a button on the card's surface prompts it to utter "Say your password" in female voice. If the voice-recognition software proves that the password is authentic (真实的), it sends its ID squawk which the server then identifies, allowing the transaction to proceed.The voice credit card is an important invention because A．it can be made cost-effectivel
B．it can leave voice message
C．it makes online shopping eas
D．it makes fraud difficul
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