程序运行时，在Text1中输入字符串，单击“开始统计”按钮，在List1中显示统计结果。函数IsLetter(x)功能，若x是字母，则返回值为字母x在字母序列“A”~“Z”中的序号，如字母“A”的序号为0，字母“D”的序号为3。若x不是字母，则返回值为-1。

Function IsLetter(x As String) As Integer

        If "A" <= x And x <= "Z" Then

            IsLetter = Asc(x) - Asc("A")

        ElseIf "a" <= x And x <= "z" Then

            IsLetter = Asc(x) - Asc("a")

        Else

                       ①        

        End If

    End Function

    Private Sub Command1_Click()

        Dim d(25) As Integer, m As Integer, n As Integer

        Dim s As String, c As String

        List1.Clear

        For m = 0 To 25                  '初始化数组d

            d(m) = 0

        Next m

        s = Text1.Text

        For m = 1 To Len(s)              '读取字符串中的字符进行判断

            c = Mid(s, m, 1)

            n = IsLetter(c)

            If n <> -1 Then    ②  

        Next m

        For m = 0 To 25                  '输出结果

            If    ③   Then

               List1.AddItem (Chr(m + Asc("a")) & ":" & d(m))

            End If

        Next m

   End Sub

回答下列问题：

Const n = 8

Dim a(1 To n) As Integer

For i = 1 To n

    a(i) = Int(Rnd * 20)

Next i

x = n \ 2

For i = x To 2 Step -1

    For j = 1 To i - 1

      If a(j) < a(i) Then

        t = a(j): a(j) = a(i): a(i) = t

      End If

    Next j

Next i

i = n

Do While i > x + 1

    k = i

    For j = i - 1 To x + 1 Step -1

      If a(j) > a(k) Then k = j

    Next j

    If k <> i Then

      t = a(k): a(k) = a(i): a(i) = t

    End If

    i = i - 1

Loop

执行完这段代码后，数组 a 中各个元素的值可能的是（  ）

执行该算法，依次输入a的值为3，5，2后，算法的输出结果为(     )

'待排序数据存储在数组a中（a（1）~a（n）），要求升序排列

For i=1To（n-1）\2

  For j=1To n-i*2

    If a  Then         '①

      t= a（j）：a（j）= a（j+ 2）：a（j+2）= t

    End If

  Next j

Next i

For i = 1 To n\2

    j = 2 * i - 1

    If a（j）> a（j+ 1）Then t = a（j）：a（j）= a（j+1）：a（j+1）= t

Next i

For i=  Step 2         '②

    t = a（i）：j = i - 1

    Do While t <a（j）

      a（j+ 1）= a（j）：j =j-1

    Loop

    a（j+ 1）= t

Next i

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