问题详情：
改错：(2019·全国卷Ⅰ改编)Modern methods of tracking polar bear populations are expensive performing consistently over a large area.
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【回答】
performing→to perform 此处是“主语＋be＋adj.＋不定式”结构，不定式用主动形式表被动含义。
知识点：非谓语动词
题型：选择题
