how to select the maximum value from multiple columns c#

Question

I have a Access DB with multiple columns, 

 

Mark1	Mark2	Mark3	Mark4	Mark5
99	87	78	78	79

 

I need to get the max value and display it in a textbox

then I need to get the avg value and display it in a text box 

 

here is my code:please help with the part as bold

connection.Open();
OleDbCommand command = new OleDbCommand();
command.Connection = connection;
string query = "select* from Students WHERE Firstname LIKE '" + tbsearchid.Text + "' OR ID1 like '" + tbsearchid.Text + "' ";
command.CommandText = query;


OleDbDataReader reader = command.ExecuteReader();
while (reader.Read()) {
 tbFname.Text = reader["Firstname"].ToString();
 tbLname.Text = reader["Lastname"].ToString();

 string course1Text = reader["Course1"].ToString(), course2Text = reader["Course2"].ToString();
 tbmulti.Text = "Course1: " + course1Text + Environment.NewLine + "Course2: " + course2Text;
 help here >> tbhighmark.Text = max Value;
 help here >> tbCourseAvg.Text = tbCourseAvg value;
}
connection.Close();