2. 外语
  3. Scientists have spent years______ into the effects of certain chemicals on the human brain with no results.

Scientists have spent years______ into the effects of certain chemicals on the human brain with no results.

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问题 Scientists have spent years______ into the effects of certain chemicals on the human brain with no results.
选项 A、studying
B、investigating
C、inspecting
D、researching
答案D
解析
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