Meaning of "drop in"

[Salman]

English Language Tests, Intermediate level

ESL/EFL Test #192 "Phrasal Verbs (D to F)", question 3

I ......... on my friend yesterday to surprise her.

(a) dropped
(b) dropped over
(c) dropped in

English Language Tests, Intermediate level

ESL/EFL Test #192 "Phrasal Verbs (D to F)", answer 3

I dropped in on my friend yesterday to surprise her.

Correct answer: (c) dropped in
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i want to find the meaning of "drop in"

Salman

[Mister Micawber]

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The correct answer is: I dropped in on my friend.

Drop in = visit informally and spontaneously.
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Native English teacher at Mister Micawber's

[Andrey]

As I know there are some more exprecions which mean visit informally and spontaneously. I'm not sure but I think it's come across. Am I right?

[Mister Micawber]

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No. 'Drop in', 'come by', 'drop by'. 'Come across' means 'casually discover'.
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Native English teacher at Mister Micawber's