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
_________________________

i want to find the meaning of "drop in"

Salman

[Mister Micawber]

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

Drop in = visit informally and spontaneously.
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_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber