VB连接数据库提示 找不到可安装的ISAM

代码如下
Private Sub Command1_Click()
Dim sql As String     'SQL语句
Dim connectionstring As String     '连接信息
Dim conn As New Connection     '数据库连接变量
Dim rst As New Recordset     '结果集
connectionstring = "Provider=Microsoft.Jet.OLEDB.4.0;dxdata Source=" & App.Path & "\考试.mdb;Persist Security Info=False"

  conn.Open connectionstring
    sql = "select * from 单选题 where 题号  = '" & th & "'"
      rst.Open sql, conn, adOpenKeyset, adLockPessimistic
     Label1 = rst.Fields("题目")
End Sub