Consider this query:

select *

from Temps T1, Temps T2

where T1.time = T2.time and T1.temp <> T2.temp

Which of the following statements is correct?

Relations T1 and T2 may not have the same schema.

If Temps.temp is a primary key, this query always returns empty results.

If and only if there are duplicate tuples in Temps, this query returns nothing as result.

This query will return nothing if and only if no two tuples have different temp values but the same time value.