The World Expo 2010 is being held at a time the world is looking for a new kind of balance between nature and cities．
A. when
B. where
C. which
D. that
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(答案→)A. when
B. where
C. which
D. that
解析：考查定语从句的关系词。此处time作先行词，其在定语从句中作时间状语。句意：2010世博会在一个世界正在寻找新的自然和城市的平衡的时代举行。