Which one of these functions do NOT have an inverse?

I know that it has to be the function that is 'many to one', but im having trouble finding that one.

Heres the four answers to choose from.

A) f(x) = 1 / (x - 1)

B) f(x) = x^3 + 1

C) f(x) = x^2 + 3 (where x is bigger/equal to 0)

D) f(x) = x^3 - 2x

I sort of worked out it it's going to be one to do with powers and negative numbers, which can be used to make the same product, but im can't seem to work out which one. I've only used trial and error really and a bit of logic. Can someone give me a hint or even better the answer and explanation?

Thanks in advanced!

..and sorry about my lack of math script skills.