10. The key to this problem is to find the expressions of different domains, so as to evaluate the domains.

X belongs to

So you should choose D, do you understand?

draft

B=f(A)

Mapping is defined as one-to-one correspondence or many-to-one correspondence.

The first one is many to one, that is, the condition of f(a)= f(b)= 1...=f(c) is always satisfied.

The second one is many-to-one and one-to-one correspondence. Because the sizes of A, B, C and D are arbitrary, the conditions cannot be satisfied forever.

The third is that they are all one-to-one correspondence and cannot be satisfied for the same reason.

It can be seen that only one kind of function can be satisfied, that is, f(x) is a constant.

Personal opinion, I hope it will help you!