What is "Canadian geese flying in formation" means?

[Mickey]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #645 "Prepositional Phrases - In, On, Out and Under (1", question 4

He saw some Canadian geese flying ......... formation and wondered how far they had to fly to reach their destination.

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

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #645 "Prepositional Phrases - In, On, Out and Under (1", answer 4

He saw some Canadian geese flying in formation and wondered how far they had to fly to reach their destination.

Correct answer: (a) in

Your answer was: correct
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What is "Canadian geese flying in formation" means?

[Mister Micawber]

Geese of many species form lines when they fly in the sky. These lines take many forms, for instance a 'V' formation-- two diagonal lines joined at the tip to form the shape of the letter 'V'.HERE are some.
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Native English teacher at Mister Micawber's