Select the most appropriate option to fill in the blank.
Scientists all over the world are engaged in ______ how to find a vaccine for COVID-19.
A.Inspecting
B.Investigating
C.Scrutinizing
D.Researching✓ Correct
Explanation
'Researching' is the most appropriate word here as scientists conduct research to find vaccines. The phrase 'researching how to find a vaccine' correctly expresses systematic scientific inquiry, unlike the other options which don't collocate naturally with this context.
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