Georg {GAY-ORG} Cantor was one of the first mathematicians to describe a theory regarding these, while a real one of these contains all rational and irrational numbers. A real one of these is also an infinite one, since it contains an undefinable amount of numbers. A union is a combination of two or more of these, while the cardinality is the highest number of elements in one of these. For 10 points, name these mathematical entities that contain a finite or infinite amount of numbers and are denoted with curly braces.
a) Sets
b) Numbers
c) Sequences
d) Variables