She is a collective subject but at the same time she has one single centre of consciousness. (faith.org.uk)
Next come dual 2.4 - inch diameter front pipes that are joined before a pre-silencer and 2.7 - inch single centre pipe, after which the system splits into two 2.1 - inch pipes, feeding into twin 16.8 - litre silencers. (motor-prime.com)
The London insurance market is still the largest single centre for aviation insurance. (en.wikipedia.org)