Pre-seeded teams have been allotted quarter-final venues. Irrespective of the positions they finish in, they will be playing at that respective location on the given date.
Sri Lanka on 18th March (Sydney)
England on 19th March (Melbourne), now Bangladesh
Australia on 20th March (Adelaide)
New Zealand on 21st March (Wellington)
The position at which the above teams finish shall dictate their opposition from Group A. This has been done to ensure that people supporting their favourite teams do not have to wait until the last moment to book their tickets. So a Sri Lankan fan will book the Sydney match on 18th March.
You mean "Group B", right? Coz the above teams are from Group A.
Its actually wierd that Group A teams get to pick their venues well in advance. Group B has the big teams like India, Pakistan and South Africa (esp. India) which have a large fan contingent and they cannot tell where India is going to play its QF and/or SF.