The officials in the _______ at the London Heathrow Airport were very polite.

问题描述:

The officials in the _______ at the London Heathrow Airport were very polite.
A.Custom
B.Customs
C.custom
D.customs
为什么要选B

customs,复数,是海关.custom,单数,是习俗.
Customs,大写,是特指某地的海关.这里指伦敦希思罗机场的海关.