I got the same wrong (but right) answer as everyone else (Option 4 only), but learned something while testing the result.

select '2009-02-01 00:00:00:000' union

select '2009-01-31 23:59:59:999'Produces 2 rows.

select convert(datetime,'2009-02-01 00:00:00:000') union

select convert(datetime,'2009-01-31 23:59:59:999')Only 1 row inserted.

Obviously, the UNION removal of duplicates is done as a character type in the first case, then the result is converted and rounded to produce 2 identical values, while in the second case, the conversion rounds both values to be the same before removing duplicates resulting in only 1 value!