A、One that is unique and difficult. B、One that is easy to remember and unsafe. C、One that is composed of letters. D、One that has

Passwords are important. Your employer, your bank and your smartphone all require one. And, for your own security, passwords should be unique and difficult.(19)However, a lot of people choose instead something easier to remember—and easier to hack. As web security suffers from the limits of human memory,(20)some psychologists and developers are experimenting with new kinds of passwords that play to your brain’s skill for remembering faces. Science shows that random letters and numbers are tricky to remember. But faces? Those are easy. A new set of experiments from researchers explore memory’s powerful connection with images of people. They discovered that to a stranger, different pictures of the same person often appear to be those of different people. But when you look at people you know—from any angle, wearing sunglasses, whatever—you recognize them. The scientists call their system Facelock. It’s not on the market yet, but here’s how it might work:(21)A website asks you to choose a few people you recognize—the more irrelevant they are with each other, the better. A good mix, for example, might be your best friend from junior high, your cousin in Arizona, your hairdresser and your favorite folk cellist. It can’t be anyone too recognizable, or else it’s a security risk. You’d be asked to select a few different images of each of these people. Then, the next time you log in, you see a series of 3-by-3 face grids—each one displaying eight random faces and one that you’ve chosen. Find a familiar face four times in a row, and you’re in.(22)For you, choosing the person you know is simple. For an intruder, it’s a guessing game.
19. What kind of password do a lot of people choose?
20. What does the new password require the users to do?
21. What can we do to ensure a good mix of passwords in the system of Facelock?
22. What can we learn about the system Facelock?