Borel signma fields

http://i44.tinypic.com/2j4o64m.png

I really don't understand what to do, i know a borel sigma field is the set with the smallest number of fields but i dont understand how to show this :(

Denote the collection of , , the collection of , , etc... Since , each element of is in the -algebra generated by , so . Use arguments of this type to get the other inclusions.