Given the task definition and input, reply with output. In this task, you're given an article, a question which often contains a blank and four options (associated with "A", "B", "C", "D"). Your task is to find the correct answer (from the given options) for the question from the given article and return one of the options from "A", "B", "C", and "D". Do not generate anything else apart from one of the following characters: "A", "B", "C", "D". There is only one correct answer for each question.

Article: Oyster What is an Oyster card? Oyster is the easiest way to pay for journeys on the bus, Tube, tram, Docklands Light Railway (DLR), London Overground and National Rail journeys in London. You can store your travel cards, Bus&Tram Pass, season tickets and credit to pay for journeys as you go. Where to get an Oyster card? There are a number of ways for you to get an Oyster card: * At over 3,900 Oyster Ticket stops * At Tube and London Overground station ticket offices * At some National Rail stations * At London Travel information Centres * Online at tfl. gov. uk/oyster How to use an Oyster card? To pay the correct fare on the Tube, DLR, London Overground and National Rail services, you must always touch in on the yellow Oyster card reader at the start of your journey, and touch out at the end. If you don't, a maximum cash Oyster fare will be changed. When using the bus or tram, you must only touch in at the start, but not at the end of your journey. What happens if I don't visit London very often? Don't worry. Any pay as you go credit on your card will not expire , so you can keep it for your next visit or lend it to a friend. Fares Traveling by Tube from Central London (Zone l)to Heathrow (Zone 6) Adult Oyster single fare PS 4. 20 Monday to Friday 06:30 - 09:30 and 16:00 -19:00 PS 2. 70 at all other times including public holidays Adult single cash fare PS 5.00 For further information, visit tfl.gov. uk /fares.
Question: Which of the following is TRUE about Oyster cards?
Options: (A) They are limited in use to the owners themselves. (B) On National Rail services you must touch them on the reader twice. (C) They are not suitable for those who don't visit London often. (D) Oyster card Tube fares cost more on public holidays than on weekdays.
B