Relations and Sets

Question

From my understanding, One = {1,2,3,4,5...20}, and then if Two = {1}, then One won't map into Two. On that token, how come the answer is d instead of a?

Answer 1

The relation One has 20 tuples. Note that the key of One is that (it is underlined), so we are sure that each tuple has a different values for that, so the cardinality of One[that] is 20, but it could have the same (or different) values for this, so we know with certainty only that it has at least a value for this (see the example of @ypercubeᵀᴹ).

Since we know that One[this] ⊆ Two[tweedledee], we know that Two must have at least one tuple, and possibly more. Note that the fact the tweedledee is a not a key of relation Two means that there could be many tuples with the same value of this attribute.