问题补充：
Is this research center you visited the modern equipment last year?
A. where
B. that
C. the one that
D. the one where
答案：
A. where
B. that
C. the one that
(答案→)D. the one where
解析：将疑问句改为陈述句：this research center is…显然缺少先行词，必须加上代词the one来作先行词，又因定语从句不缺主语或宾语，所以用关系副词where。假若在this后加the，就选where，请想想这是为什么。
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