"As you probably know" vs. "As you certainly know"

[Makassi]

English Language Tests, Intermediate level

ESL/EFL Test #123 "English tense explanations", question 4

As you ......... know there are many tenses in English.

(a) probably
(b) certainly
(c) sometimes
(d) doubtfully

English Language Tests, Intermediate level

ESL/EFL Test #123 "English tense explanations", answer 4

As you probably know there are many tenses in English.

Correct answer: (a) probably

Your answer was: incorrect
As you certainly know there are many tenses in English.
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I'd like to know what's wrong with "As you certainly know"?

Thank You
Makassi

[Alan]

Hi,

This is a comment made by the teacher who is being polite and assuming that the student probably knows but is not certain that the student knows.

Alan
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English as a Second Language
You can read my ESL story Briefly