Question 2, choose C.

Question 3, choose a.

Question 4, choose C.

Question 5. Choose B to satisfy reflexivity.

Question 6. Choose an even number.

Question 7, choose D.

Question 8, choose C, this question is uncertain, reflexivity must be satisfied, symmetry and transitivity are not established.

Question 9, choose D.

Question 10, choose B.

Question 1 1, choose a tautology.

Question 12, choose C.

Question 13, c satisfies reflexivity.

Problem 14, the choice of d dual graph is not necessarily a plane graph.

Question 15, choose b to satisfy the commutative law.

Question 16, choose a.

Question 17, option b must be equal.

Question 18, choose a.

Problem 19, select symmetric closure.

Question 20, choose C.