Collections

I found this question from mock exame.
Which of the following can contain duplicate elements.
a)collection
b)List
c)Set
d)Map
Given answers is 'a' and 'b'.
How is 'a' is a answer.
Thanks
Satish

Hi Satishind,
API says "Some collections allow duplicate elements and others do not "
So I guess, you cannot really say a) is true.
I have come across this kind of answer before... and I wondered myself....
correct me if I am wrong here.. thanks