Succor me! (Usage of "since then")

[Nameless]

Which one is correct?
.............since then.
a. Four years have passed
b. It has passed four years

If the answer is "a", I think "Four years" is a singular subject. So it must be "Four years has passed"

[Yankee]

Hi Nameless

Option (a) is correct. In my opinion, most native speakers would also usually view it as plural (and thus use the plural form of the verb) rather than view it as a single "unit of time". However, I would not rule out the singular usage on occasion.
.
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[Jamie (K)]

The correct answer is "a". "Four years" is a plural subject (1 year, 2 years...), so it must be, "Four years have passed."