为什么.net用C#往Access数据库写不进数据，但删除或读取都可以

<script language="C#" runat="server">
void btn(object sender,EventArgs e)

{
string sid=textbox1.Text;
int stihao=0,flag=0; //数据库中若无数据，flag不会产生循环赋值，必须赋初值0
string ti_mulu;

string strc = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = ";
strc+=Server.MapPath("student.mdb"); //使用相对路径
//C:\\Inetpub\\wwwroot\\RWdatabase\\student.mdb"; //原来的绝对路径
OleDbConnection olecon=new OleDbConnection(strc);
olecon.Open();

OleDbCommand comd = new OleDbCommand();
comd.Connection = olecon;
comd.CommandText = "select * from biao";

OleDbDataAdapter DA = new OleDbDataAdapter("select * from biao",olecon);
// DA.SelectCommand = comd;

//DA.InsertCommand = combuilder.GetInsertCommand(); //这行好象没用