C # Return Values of a Thread

I created a database connection class that I want to use Thread to make queries faster.

In functions ExecutarSelect and ExecutarSQLPiece I have returns, but I could not get those returns through the thread.

I did a search but could not get the solution, could someone help me?

Thank you very much!

// Instantiating and calling thread execution

        ThreadBD thBD = new ThreadBD();
        thBD.comandoSQL = "SELECT acessoRapido FROM dbo.parametrosUsuario";
        Thread th = new Thread(thBD.executarSQLPiece);
        th.Start();

// DataBase Connection Class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using System.Data.SqlClient;
using System.Windows.Forms;
using System.Threading;

namespace H7Transporte
{

    class ConexaoBD
    {
        //string connection;
        DataTable tabela = new DataTable();
        string retorno;
        public ConexaoBD()
        {
        }

        public void ExecutarQuery(string comandoString)
        {
            string connectionString = "Data Source=.\SQLServer;Initial Catalog=H7Transporte;Integrated Security=True";
            SqlConnection cnn = new SqlConnection(connectionString);
            try
            {
                cnn.Open();
                SqlCommand comando = new SqlCommand(comandoString, cnn);
                comando.ExecuteNonQuery();
            }
            catch (Exception)
            {
                MessageBox.Show("Não foi possivel executar seu comando. O Query pode estar errado ou o retorno é nulo ");

            }
            finally
            {
                cnn.Close();
            }
        }

        public DataTable ExecutarSelect(string comandoString)
        {
            string connectionString = "Data Source=.\SQLEXPRESS;Initial Catalog=teste2;Integrated Security=True";
            SqlConnection cnn = new SqlConnection(connectionString);

            try
            {
                cnn.Open();
                SqlDataAdapter Preencher = new SqlDataAdapter(comandoString, cnn);
                Preencher.Fill(tabela);
            }
            catch (Exception)
            {
                MessageBox.Show("Nenhuma informação foi encontrada. Seu Query pode estar errado ou o retorno é nulo ");
            }
            finally
            {
                cnn.Close();
            }
            return tabela;

        }

        public string sqlPiece(string comandoString)
        {
            string connectionString = "Data Source=.\SQLServer;Initial Catalog=H7Transporte;Integrated Security=True";
            SqlConnection cnn = new SqlConnection(connectionString);

            try
            {
                cnn.Open();
                SqlCommand comando = new SqlCommand(comandoString, cnn);
                SqlDataReader sqlReader = comando.ExecuteReader();

                sqlReader.Read();
                retorno = sqlReader.GetValue(0).ToString();

            }
            catch (Exception)
            {
                MessageBox.Show("Nenhuma informação foi encontrada. Seu Query pode estar errado ou o retorno é nulo");
            }
            finally
            {
                cnn.Close();
            }

            return retorno;
        }
    }

    class ThreadBD
    {
        public string comandoSQL { get; set; }
        public string retorno { get; set; }

        public void executarQuery()
        {
            ConexaoBD conecta = new ConexaoBD();
            conecta.sqlPiece(comandoSQL);
        }

        public void executarSelect()
        {
            ConexaoBD conecta = new ConexaoBD();
            conecta.sqlPiece(comandoSQL);
        }

        public void executarSQLPiece()
        {
            ConexaoBD conecta = new ConexaoBD();
            conecta.sqlPiece(comandoSQL);
        }
    }
}