Difference between surely and certainly

Test No. [color=blue]incompl/elem-24 “Shopping”, question 2

you must know what we need?

(a) Certainly
(b) Surely
© Definitely
(d) Absolutely

Test No. [color=blue]incompl/elem-24 “Shopping”, answer 2

Surely you must know what we need?

Correct answer: (b) Surely

Your answer was: [color=red]incorrect
Certainly you must know what we need?

Hello everybody!!!
I have just started with the tests.
In this one I can?t get the difference between certainly and surely. Would you help me please?

In this context surely is when you believe that’s what the other person wants/believes but certainly is to show that this is what you believe/want.

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