More wild tigers have been seen in the forest around this area, there used to be very few

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A. when
(答案→)B. where
C. what
D. which
解析：定语从句缺少地点状语in the forest，所以其关系词必须是where或in which.
答案A在定语从句中做时间状语； C what不能引导定语从句； D的which只能直接代替先行词the forest，但是它不能在该定语从句中直接充当成分。