What does 'cream always rises to the surface' mean?

[Zishuli]

English Language Tests, Intermediate level

ESL/EFL Test #336 "English Slang Idioms (57)", question 10

He told his grandfather that he had improved his school grades from a C to an A average, and his grandpa replied, " See? I told you ......... always rises to the surface."

(a) cream
(b) bubbling
(c) wood
(d) syrup

English Language Tests, Intermediate level

ESL/EFL Test #336 "English Slang Idioms (57)", answer 10

He told his grandfather that he had improved his school grades from a C to an A average, and his grandpa replied, " See? I told you cream always rises to the surface."

Correct answer: (a) cream

Your answer was: correct
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Hi,
Can anyone explain to me what's the meaning of " cream always rises to the surface" ?

Thanks.

[Mister Micawber]

.
Excellence always becomes evident.
.
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Native English teacher at Mister Micawber's