First of all, AD is definitely wrong, n is natural number set, 0 ∈ n; An empty set is a set, and the symbol ∈ cannot be used. What is the correct expression? Contained in {0}

C is right, {? } is a set composed of empty sets, in other words, an empty set is an element of this set, it is not an empty set. Really included in {? } Of course it's right. What else can you say? ∈{? }, this is also a fact.

B there is something wrong. First of all, this is not a conclusion at all. Secondly, it is also a collection, which can be regarded as a collection represented by enumeration. Its element is the equation x 2-x = 0, which is a set of equations.