'Happen to know' vs 'know by chance'

[Edita]

English Language Tests, Intermediate level

ESL/EFL Test #251 "Polite Indirect Questions & Statements Test (1)", question 4

New York is such a big city. ......... where the subway station is?

(a) Do you happen to know
(b) You tell me
(c) You know

English Language Tests, Intermediate level

ESL/EFL Test #251 "Polite Indirect Questions & Statements Test (1)", answer 4

New York is such a big city. Do you happen to know where the subway station is?

Correct answer: (a) Do you happen to know
_________________________

help me!

Edita

[Mister Micawber]

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What seems to be the problem?
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Native English teacher at Mister Micawber's

[Forastero]

I know the correct answer is a, because is the only option with the auxiliary verb do. Although i don't know what the expression "happen to know" means

Can you explain me?

I learned to ask: Could you tell me...? Do you know...? but not this option showed here

[Yankee]

[Forastero]

Oh, thanks a lot Amy. It's a very useful courtesy form. In spanish it's very common: "por casualidad" and I didn't know how to say it in english