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(TextText) TextText = Str(【 】) End Sub Function fun1(a As Long，b As Long)As Long Dim r As Long Dim t As Long If【 】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 【 】 End Function