'On the microscope' vs 'under the microscope'

[pulidogato]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #653 "Prepositional Phrases - In, On, Out and Under (26)", question 4

The Bureau of Internal Affairs has had my department ......... the microscope for a whole week now; searching for any signs of wrongdoing or mistakes.

(a) in
(b) on
(c) out
(d) under

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #653 "Prepositional Phrases - In, On, Out and Under (26)", answer 4

The Bureau of Internal Affairs has had my department under the microscope for a whole week now; searching for any signs of wrongdoing or mistakes.

Correct answer: (d) under
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Hello, why can´t be "on". tanks

pulidogato

[Mister Micawber]

'On the microscope' is not an English collocation. Things to be examined are placed under a microscope. Only your hat could be placed on your microscope.
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Native English teacher at Mister Micawber's