VB替换htm文件的有关问题，为什么不是覆盖而是追加呢？

VB替换htm文件的问题，为什么不是覆盖而是追加呢？？
DownloadFile   "     http://www.baidu.com/s?wd= "   +   GetaLine(RichTextBox2,   i   -   1)   +   "&cl=3 ",   mulu   &   qianzhui.Text   &   " "   &   i   &   ". "   +   houzhui.Text


Dim   S1   As   String
Dim   d   As   String
Open   mulu   &   qianzhui.Text   &   " "   &   i   &   ". "   +   houzhui.Text   For   Input   As   #3

While   Not   EOF(3)
Line   Input   #3,   S1
d   =   d   &   S1   &   vbCrLf

Close   #4
d   =   Replace(d,   "百度 ",   "专家门诊 ")
Open   mulu   &   qianzhui.Text   &   " "   &   i   &   ". "   +   houzhui.Text   For   Output   As   #4
Print   #4,   d

Close   #4

通过下载下载好了100个文件要对这100个文件进行替换，后来发现替换第一个是正确的   从   第2个往后就开始出现问题   。。   2.htm是2个页面的内容   3.htm是三个页面的内容......生成100个的时候文件已经2.10MB了     100个页面的内容！！请老师帮忙  
请帮忙如何只覆盖不追加呢？

[解决办法]
Dim S1 As String
Dim d As String


DownloadFile " http://www.baidu.com/s?wd= " + GetaLine(RichTextBox2, i - 1) + "&cl=3 ", mulu & qianzhui.Text & " " & i & ". " + houzhui.Text

If d = " " Then
Open mulu & qianzhui.Text & " " & i & ". " + houzhui.Text For Input As #3

Do Until EOF(3)
Line Input #3, S1
d = d & S1 & vbCrLf
Loop

Close #3
d = Replace(d, "百度 ", "专家门诊 ")
End If

Open mulu & qianzhui.Text & " " & i & ". " + houzhui.Text For Output As #4
Print #4, d
Close #4