Typical has functions are not perfect because
a. they allow collisions
b. none of the other answers
c. both of the answers
d. they allow more than once hash key to map to the same index

D) they allow more than once hash key to map to the same index

Explanation:

 Typical hash function contain infinite domain like bit sequence with fixed length and have some finite range. It also implement byte string of some arbitrary length.

In many cases, hash function are design with permutation mapping between the range and identical size domain. Permutation are also known as one to one mapping. Mixing property are one of the desired property of hash function.

Therefore, it allow more than one hash key to map the same index.  

try this:

go tos official website.

click on your profile pic at the top right of the screen and click 'edit profile'.

click on "preferences" and press "year of birth".

and then just maybe try to reset whatever error made before by typing in the correct age details again. hope this !