程序功能：在文本框Text1中输入课程号，单击“查询”按钮Command1,在列表框List1中显示选修本课程的学生信息。如果没有找到，在列表框中显示“该课程当前无人报名”。按此要求将程序补充完整。

Private Sub command1_click()

Dim xh(1 to 40),xm(1 to 40) As String   '学号和姓名的数组

Dim i,num as integer

Dim conn As New ADODB.Connection, rs As New ADODB.Recordset

Dim strSQL As String

conn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + App.Path + "\student.accdb"     '打开到数据库的链接

 conn.Open

strSQL ="select xh,xm from data where '课程=&(Text1.Text) & '" '设置查询的SQL语句

 Set rs.ActiveConnection = conn      '设置rs的ActiveConnection属性，指定与其关联的数据库链接

  rs.Open strSQL   '打开记录集，将从表student中读取的结果集保存到记录集rs中

  List1.Clear

num=0

Do while not rs.EOF

             ①       

    xh(num)=rs.Fields("xh")

      xm(num)=rs.Fields("xm")

  rs.MoveNext

loop

rs.Close

  conn.close

set rs=nothing

set conn=nothing

For  i=1 to num

      ②     

Next i

if num=0 then list1.additem("该课程当前无人报名")

End Sub

Private Sub Command1_Click()

Dim a as Integer

a=Val(Text1.text) ‘将文本框中的数字文本转换成数值

a=2*a+1

Text1.text = StrA.

End Sub

程序运行时在文本框Text1中输入1，连续两次单击命令按钮Command1后，Text1中显示的内容是：（   ）

图a

s = ""

For i = 1 To Len(Text1.Text)

   s = s + Mid(Text1.Text, Len(Text1.Text) - i + 1, 1)

Next i

若文本框Textl中输入“a1b2c3”,执行程序后，变量s的值为（  ）

s = Text1.Text

List1.Clear

len1 = Len(s)                  ‘①

For i = 1 To len1              ‘②

For j = 1 To len1- i           ‘③

List1.AddItem Mid(s, j, i)  ‘④

Next j

Next i

运行时发现未输出所有子串，则修改（  ）

Dim a(1 To 15) As Integer

Private Sub Form_Load()

  Dim i As Integer, t As Integer

  Dim s1 As String, s2 As String

  Randomize

  s1 = ""

  For i = 1 To 15

    a(i) =  ①    

    s1 = s1 + Str(a(i))

  Next i

  Text1.Text = s1

End Sub

Private Sub Com1_Click()

  s2 = ""

  For i = 1 To 15

    s2 =  ②    

  Next i

  Text2.Text = s2

End Sub