求问，更改数据库，asp读取mdb,报错是，UPDATE 语句的语法错误。”

int selnum=1;//
//是否第一次读取该问题

//题目显示
protected void Page_Load(object sender, EventArgs e)
{

}

//读取答案文件


protected void Request_Click(object sender, EventArgs e)
{


    string strSQL = "Provider=Microsoft.jet.OleDb.4.0;data source=" + Server.MapPath("database.mdb");
    OleDbConnection conn;
    OleDbCommand cd; 
    conn = new OleDbConnection(strSQL);
    conn.Open();
    cd = conn.CreateCommand();
    cd.CommandText = "Update Question Set Add ='" + answer.Text.Trim() + "'where Number='" + selnum.ToString() + "'";

    cd.ExecuteNonQuery();
    conn.Close();


    Response.Write("<script>alert('回答成功');location.href='question_teacher.aspx';</script>");
}


protected void Question_TextChanged(object sender, EventArgs e)
{


}

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
    question.Text = GridView1.SelectedValue.ToString();
    selnum = GridView1.SelectedIndex+1;

}



    麻烦大佬们帮忙看看，运行到更新数据库就报错，找不到原因

Number是不是字符串类型
Add Question这个表、字段名字是否正确
where前面少空格