Read the article and select the best answer. Article: Visiting London can be an expensive trip, so establishing a budget and finding bargains when you arrive will help you get around London on the cheap. The following steps can help you find cheaper alternatives for transport for your London trip. l Familiarize yourself with the city's boroughs and specifically the area where you will be staying. You can use a street map, or an online map search engine, to establish where in the city you will be staying and the distance you are from the major tourist attractions you want to visit. l Find a local provider of Transport for London tickets or travel cards, since your first bargain purchase will be an Oyster Card. Traveling around London with an Oyster Card will give you access to the cheapest option of transportation methods and the Oyster Card is accepted citywide on buses, boats and the London Underground, known by the locals as "the tube". l Seek out your nearest Tube Station. The Tube is the cheapest and most reliable form of transportation and runs from around 6 a.m. until around 1 a.m. ..., depending on the line or the station. There are 287 tube stations in London that accept the Oyster Card, with a high concentration in central London. If you are staying anywhere in central London, there will be a tube station less than 10 minutes away. The Transport for London website offers a tube station search facility to find the nearest subway station to any location in the city. When you find a tube station, you can easily locate the closest bus stop by using the comparison bus-tube maps at the station's entrance. l Follow the Transport for London Tube Guide to your destinations and always plan ahead. Making a plan for transport will save you unnecessary expenses, wasted trips on the tube and doubling back on your journey . Question: What's the author's purpose in writing the passage? Options: A: To introduce the public transport system in London. B: To show readers how to use the Oyster Card. C: To prove the subway in London is easy to use. D: To offer tips for saving transport costs in London.
The answer to this question is:
D