主题：关于随机抽取记录问题

假如在题库中抽取100个记录，其中50个是单选题，50个是多选，我想实现抽取20个单选，10个多选，如在单选题中抽取时遇到有2个问的题时，总抽选题数就少1个，保持问题总数为20个，如何实现，请各位专家老师，指点一下，谢谢

Dim cnt As Integer = DataTables("题库").DataRows.Count
Dim ids1 As New List(of Integer)
Dim ids2 As New List(of Integer)
For i As Integer = 0 To cnt -1
    ids1.add(i)
Next
For i As Integer = 0 To cnt - 1
    ids2.Add(ids1(rand.Next(0,ids1.count)))
Next
Tables("题库").StopRedraw()
DataTables("题库").ReplaceFor("选择",False)
For i As Integer = 0 To args(0) -1 '100为要抽取的行数
    DataTables("题库").DataRows(ids2(i))("选择") = True
    If Functions.Execute("Q_Number",DataTables("题库").DataRows(ids2(i))("题目内容"),"（？）" ) > 1 Then
     i += 1
    msgbox(i)
    End If   
Next
Tables("题库").Filter = "[选择] = True"
Tables("题库").ResumeRedraw()