Prepositions of time

[sveta]

English Language Tests, Intermediate level

ESL/EFL Test #206 "Prepositions of Time and Date (4)", question 10

......... 2005, I finished my degree and now I'm working with a small accounting firm.

(a) On
(b) In
(c) At

English Language Tests, Intermediate level

ESL/EFL Test #206 "Prepositions of Time and Date (4)", answer 10

In 2005, I finished my degree and now I'm working with a small accounting firm.

Correct answer: (b) In

Your answer was: incorrect
At 2005, I finished my degree and now I'm working with a small accounting firm.
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Could you please explain me what is the rule for this sentence?

Sveta

[Mister Micawber]

.
The general guideline is:

At a time: at 8:30, at noon
On a day: on Tuesday, on November 22nd
In a month, year, etc: in May, in 2006, in the 21st century.
.
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Native English teacher at Mister Micawber's