在下列个数中，$\sqrt{3}$，﹣2，0，$\sqrt{16}$，$\frac{22}{7}$，3.1415，$\sqrt[3]{-27}$，﹣$\frac{\mathrm{\pi }}{2}$，$\sqrt[3]{9}$无理数的个数为（　　）

阅读理解

Oyster

What is an Oyster card?

    Oyster is the easiest way to pay for journeys on the bus, Tube, tram, DocklandsLight Railway (DLR), London Overground and National Rail journeys in London Youcan store your travel cards, Bus & Tram Pass, season tickets and credit topay 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 andNational Rail services, you must always touch in on the yellow Oyster cardreader at the start of your journey, and touch out at the end. If you don't, amaximum cash Oyster fare will be charged. When using the bus or tram, you mustonly 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 notexpire (过期), so you cankeep it for your next visit or lend it to a friend.

Fares

Traveling by Tube from Central London (Zone l)to Heathrow (Zone6)

Adult Oyster single fare

£ 4. 20 Monday to Friday 06:30 - 09:30 and 16:00 - 19:00

£ 2. 70 at all other times including public holidays

Adult single cash fare £ 5.00

For further information，visit tfl.gov.uk /fares.