【加试题】如图所示，a 、b 、c 三个数比较大小，单击“求最大值”按钮，在文本框Text4中输出三者之间最大的数，选出不能实现该功能的程序段（   ）

A：If a >= b And a >= c Then Text4.Text = Str(a)If b >= a And b >= c Then Text4.Text = Str(b)If c >= a And c >= b Then Text4.Text = Str(c)

B：Max = aIf b > Max Then Max = bIf c > Max Then Max = cText4.Text = Str(Max)

C：If a > b And a > c Then    Text4.Text = Str(a)ElseIf b > a And b > c Then    Text4.Text = Str(b)Else    Text4.Text = Str(c)End If

D：If a > b Then   If a > c Then Text4.Text = Str(a) Else Text4.Text = Str(c)Else   If b > c Then Text4.Text = Str(b) Else Text4.Text = Str(c)End If