Sandy went to Tibet by train this summer for the first time. She had an unusual .
A. experience
B. experiment
C. knowledge
D. condition
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(答案→)A. experience
B. experiment
C. knowledge
D. condition
解析：experience经验，经历；experiment实验，尝试；knowledge 知识，学问；condition条件，环境。句意：这个暑假桑迪第一次乘火车去了西藏。她有一段不寻常的经历。结合语境可知选A。