下列程序的功能是计算两个自然数的最大公约数，界面如下图所示。给定的程序不完整，将程序补充完整。

Private Sub Command1_Click()
Dim x As Long， y As Long，i As Long
x=Val(Text1．Text)
y=Val(Text2．Text)
Text3．Text = Str( 【13】 )
End Sub
Function fun1(a As Long，b As Long)As Long
Dim r As Long
Dim t As Long
If 【14】 Then
t = a
a = b
b = t
End If
r = a Mod b
Do While r ＜＞ 0
a = b
b = r
r = a Mod b
Loop
【15】
End Function