Read the following article and answer the question. Article: The London Underground (The Tube) The main source of public transport in London revolves around the Underground (or the Tube as it is known to Londoners). This network of 12 lines can get you to most places in the center of the city quickly. It's most necessary to get an Oyster Card or a Travelcard. Single tickets are priced starting at the intentionally absurd price of PS 4.80 (Zone 1-3), if you pay cash. Using an Oyster card, a single fare is PS 2.30 if you are traveling within the central Zone 1. On hot days it is also advisable to take a bottle of water with you as Underground trains are not air-conditioned. Last trains leave central London at around 00:30 weekdays, 23:30 Sundays. First trains leave the suburbs at around 5:00. The Bus System Outside the center of London, Tube stations are farther apart, so buses help fill the gaps. Also, the bus offers a cheaper alternative, even if it is a slower journey. Cash fares for London buses have been abolished -- you cannot pay cash. A bus fare costs PS 1.50 if using only buses, the fare is capped at PS 4.40 per day for Oyster or contactless users. River Services There are a number of different routes along the River Thames. The faster commuter services operate all day from Greenwich Pier to Embankment and from Putney and Chelsea harbour to Blackfriars during Peak Hours only. These routes will pass a number of places of interest including the Houses of Parliament and London Bridge. A return fare from Putney to Blackfriars will cost about 12 pounds. National Rail Once you leave Central London or if you are traveling South of the River Thames, the best public transport option will often be National Rail. There are numerous connections to the Rail System from the Tube. Travelcards can be used for travel on the National Rail (but not the Heathrow Express). Oyster cards can be used up to Zone 6 except certain services including Heathrow Express, Heathrow Connect and HSI. Question: When taking the London Underground, _ . Answer:
Answer:
you are advised to take a bottle of water along on hot days