If passengers have anything that needs to be declared, they should go through ___.

Customs 意指「海關」。海關負責查驗出入境旅客攜帶的物品、管制違禁品並課徵關稅。當旅客攜帶超額現金、應稅物品或管制物品時，必須向海關申報（declare），故為正解。