When we can use 'Drop in and drop in on"?

[lam Tung]

English Language Tests, Intermediate level

ESL/EFL Test #156 "Idioms with the phrasal verb drop", question 3

It is not necessary to attend every class in order to participate, so please ......... and join the fun!

(a) drop off
(b) drop in on
(c) drop behind
(d) drop in

English Language Tests, Intermediate level

ESL/EFL Test #156 "Idioms with the phrasal verb drop", answer 3

It is not necessary to attend every class in order to participate, so please drop in and join the fun!

Correct answer: (d) drop in

Your answer was: incorrect
It is not necessary to attend every class in order to participate, so please drop in on and join the fun!
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hi!
pls tell me when we can use 'Drop in and drop in on".
Thanks
lam Tung

[Conchita]

While 'drop in' can be used without an object, 'drop in on' needs one to complete the sentence.