填空题

下列程序的功能是把文件C:\a1.txt复制成C:\a2.txt，请填空。
Private Sub Form C1ick()
Dim ch As String
Open "C:\a1.txt" For ________
Open "C:\a2.txt" For ________
Do While Not ________
ch＝Input(1,10)
Print #20,ch;
Loop
Close #10,#20
End Sub