题目内容：
下列程序的功能是计算两个自然数的最大公约数，如图1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
参考答案：【答案仅供学习，请勿对照自行用药等】