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找数对。已知一数组 a 中有 9 个元素，在文本框 Text1 中输入一个正整数 key，单击“找数对” Command1 按钮，则在数组 a 中搜索是否有一对数的和等于 key，若有，在标签 Label2 中输出最先找到的两个数，若无则输出“没有找出数对”。运行界面如图所示：
 
（1）根据程序，若文本框 Text1 中输入内容为 17，则标签 Label2 中输出的内容是____________。
（2）请划线处填入合适的代码。
①__________________
②__________________
（3）请改正加框处语句的错误。
改正：__________________
Const n = 9
Dim a(1 To n) As Integer,i As Integer, j As Integer
Private Sub Form_Load()
Dim s As String, k As Integer
a(1) = 9: a(2) = 13: a(3) = 11: a(4) = 3: a(5) = 20
a(6) = 6: a(7) = 18: a(8) = 15: a(9) = 8
For i = n To 2 Step -1
k = i
For j = ①              
If a(j) < a(k) Then t = a(j): a(j) = a(k): a(k) = t
Next j
       '③改错
Next i
Label1.Caption = s End Sub
Private Sub Command1_Click()
Dim L As Integer, R As Integer, m As Integer, key As Integer
key = Val(Text1.Text)
For i = 1 To n - 1
L = 1: R = n
Do While L <= R
m = (L + R) \ 2
If a(i) + a(m) = key Then
Label2.Caption = Str(a(i)) & Str(a(m))
Exit For
ElseIf ②             Then
L = m + 1
Else
R = m - 1
End If
Loop Next i
If L > R Then Label2.Caption = "没有找到数对"
End Sub