Ensure vs Insure vs Assure vs Reassure

[Duc]

Hi,
I'm having trouble with synonyms.Here is one of that kind:
Choose the best answer.
He set one alarm clock for five o'clock and the other for five past so as to .......
A assure B. ensure C.insure D.reassure
The correct answer is B but why can't i choose the others?
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Don't see your shade think you are great

[Mister Micawber]

.
They don't have the correct meaning of 'make certain'. Have a look in a DICTIONARY.
.
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Native English teacher at Mister Micawber's

[Duc]

Hey,even in the Cambridge dictionary,''assure'' means ''make certain''.Can you give me a clearer explanation?
_________________
Don't see your shade think you are great

[Mister Micawber]

.
OK, I'll approach them with my native understanding:

A assure -- we assure persons
B. ensure -- we ensure for ourselves
C.insure -- we buy insurance
D.reassure -- we assure persons again.

These, I think, are the normal uses, though I see from the dictionaries that 'make certain of' does indeed figure in several definitions.
.
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Native English teacher at Mister Micawber's

[Duc]

Thank you.
_________________
Don't see your shade think you are great