流程图中的循环结构有（    ）

A．一个入口和一个出口	B．一个入口和两个出口
C．一个入口和三个出口	D．两个入口和两个出口

下列流程图属于循环结构的有（    ）。

A．①④

B．③④

C．②④

D．①③

小李编写一个统计学生选考科目的 VB 程序，算法的基本思想是：检查每位学生选课组合的编码是否有效，对有效选择结果进行科目统计，根据各科占有效人数的百分率求出各科排名｡程序运行时，单击“显示”按钮 Command1，学生选课数据显示在列表框 List1 中，各科编号及对应科目名称显示在列表框 List2 中，单击“统计”按钮 Command2，选课结果按各科百分率由高到低显示在列表框 List3 中，程序运行结果如图所示｡

现上述功能的 VB 程序如下，请回答下列问题：
（1）学生选课的编码数据存储在数据表中，其字段名为__________________｡
（2）请在划线处填入合适的代码｡
Dim ach(1 To 500) As String  '存储各学生选课编码
Dim bno(1 To 10) As Integer '存储各科目编号
Dim bname(1 To 10) As String '存储各科目名称
Dim bnum(1 To 10) As Integer  '存储各科目被选次数
Dim bmc(1 To 10) As Integer '存储各科目被选的名次
Dim n As Integer, num As Integer
Private Sub Command1_Click()
Dim i As Integer, km As String
km = "物理化学生物政治历史地理技术"
Dim cn As New ADODB. Connection, rs As New
ADODB. Recordset, sql As String
cn. Open "provider=Microsoft. Jet. oledb. 4. 0;Data Source =" + App. Path + " \students. mdb"
rs. Open "select ∗ from choose", cn
Do While Not rs. EOF
n = n + 1 : ach(n) = rs. Fields("subject") : rs. MoveNext
Loop
rs. Close: cn. Close
Set rs = Nothing
Set cn = Nothing
For i = 1 To 7
bno(i) = i: bname(i) = Mid(km, (i - 1) ∗2 + 1, 2)
List2. AddItem Str (bno (i)) + " - - -" +bname(i)
Next i
End Sub
Private Sub Command2_Click()
Dim i As Integer, j As Integer, k As Integer
Dim c As String, per As Integer
For i = 1 To n
If check(ach(i)) =True Then
For j = 1 To 3
c = Mid(ach(i), j, 1)
bnum(Val(c)) = ①_________
Next j
num = num + 1
End If
Next i
i = 1
Do While i <= 7  '该循环得到各学科被选人数的名次
bmc(i) = 1
For j = 1 To 7
If bnum(j) > bnum(i) Then ②__________
Next j
i = i + 1
Loop
For i = 1 To 7
For j = 1 To 7
If ③_________Then
per = Int(bnum(j) ∗ 100 / num ∗ 100)/ 100
List3. AddItem Str(bno(j)) + "---" +bname(j) + "---" + Str(per) + "%"
End If
Next j
Next i
End Sub
Function check(s As String) As Boolean
'代码略，若学生选科代码有效，函数返回 True，否则返回 False
End Function

   将以下计算50以内奇数和S的值（s=1+3+5+…+49）的程序段补充完整.

    private sub form_click( )
dim i as integer, s as integer
s=①__________
for i=1 to 50②_________
③_________
Next i
print “50以内奇数和=”; s
end sub

有如下程序段：
tail = 6：i = 1：r = Val(Text1.Text)
Do While i < r
For j = tail To i + 1 Step -1
If a(j) > a(j - 1) Then
t = a(j): a(j) = a(j - 1): a(j - 1) = t
End If
Next j
i = i + 1
For j = i To tail - 1
If a(j) < a(j + 1) Then
t = a(j): a(j) = a(j + 1): a(j + 1) = t
End If
Next j
tail = tail - 1
Loop
数组元素a(1)到a(6)的值依次为“73、56、28、61、44、92”，若在文本框Text1中输入“2”，则经过该程序段“加工”后，数组元素a(1)到a(6)的值依次为

A．73,61,56,92,44,28	B．92,73,56,61,44,28
C．92,73,61,56,28,44	D．92,73,61,56,44,28