In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.
One example: Article: There are a lot of public libraries in Britain. These libraries have more than one hundred and fifteen million books, not including those libraries of public schools. Public libraries not only lend books, music records but also help children, patients in hospitals and others to get special play readings, film shows and so on. The first public libraries were founded in about the year 1850. Today's public libraries usually have a reading-room, a lending room and a reference room. The greatest and most famous library in Britain is that of the British Library in London. It was founded in 1973 and has over 10, 000, 000 books. It has nearly every magazine, newspaper and book published in Britain and other parts across the world.
Answer: 10,000,000 or less.
Solution is here: How many books are there in the British Library?
Explanation: In the Article, 10,000,000 is mentioned for the number of books available in the British Library, so this is a good question.

Now, solve this: Article: Visitor Oyster cards are electronic smartcards that come fully charged with credit. Whether you're making a one-off trip to London or you're a regular visitor, using an Oyster travel smartcard is the easiest way to travel around the city's public transport network. Simply touch the card on the yellow card reader at the doors when you start and end your journey. Advantages of a Visitor Oyster Card A Visitor Oyster card is one of the cheapest ways to pay for single journeys on the bus, Tube, DLR, tram, London Over-ground and most National Rail services in London: * Save time----your card is ready to use as soon as you arrive in London. * It's more than 50% cheaper than buying a paper travel card or single tickets with cash. * There is a daily price cap----once you have reached this limit, you won't pay any more. * Enjoy special offers and promotions at leading London restaurants, shops and entertainment venues----plus discounts on the Emirates Air Line cable car and Thames Clippers river buses. Buy a Visitor Oyster card Buy a Visitor Oyster card before you visit London and get it delivered to your home address. A card costs PS3 (non-refundable) plus postage. Order online and arrive with your Oyster in hand! You can also buy a Visitor Oyster card from Gatwick Express ticket offices at Gatwick Airport Station and on board Eurostar trains travelling to London. Add Credit to Your Visitor Oyster Card You can choose how much credit to add to your card. If you are visiting London for two days, you can start with PS20 credit. If you run out of credit, add credit at the following locations: * Touch screen ticket machines in Tube, DLR, London Over-ground and some National Rail stations. * Around 4,000 Oyster Ticket Stops found in newsagents and small shops across London. * TFL Visitor and Travel Information Centers. * Tube and London Over-ground station ticket offices. * Emirates Air Line terminals.
Answer: Before you leave home.
Solution:
When can you use your Visitor Oyster Card?