For each blank, choose the best answer from the four choices and write down on the answer sheet.
(21) is a six bytes OSI layer 2 address which is burned into every networking
device that provides its unique identity for point to point communication.
(22) is a professional organization of individuals in multiple professions which focuses on effort on lower-layer protocols.
(23) functions with two layers of protocols. It can connect networks of different speeds and can be adapted to an environment as it expands.
(24) is the popular LAN developed under the direction of the IEEE802.5.
(25) is the popular backbone technology for transmitting information at high speed with a high level of fault tolerance which is developed under the direction of ANSI.

A．ISO

B．ANSI

C．CCYIT

D．IEEE

完成下列问题：

考生文件夹下有一个工程文件sjt3.vbp，其中的窗体上有一个名称为Cmd的命令按钮控件数组；有一个名称为Image1的图像框。请画一个名称为Timer1的计时器，时间间隔为3s，初始状态为不可用，如图3所示。
[*]
程序功能如下：
单击“前进”按钮，则Timer1的状态变为可用，且在图像框中显示3s黄灯(图像文件为考生文件夹下的yellow.ico)后，显示绿灯(图像文件为考生文件夹下的green.ico)直至下次单击某个命令按钮。
单击“停止”按钮，则Timer1的状态变为可用，且在图像框显示3s黄灯后，显示红灯(图像文件为考生文件夹下的red.ico)直至下次单击某个命令按钮。
单击“结束”按钮，则结束程序执行。
在横线处填写正确的内容，以实现上述程序功能。
注意：考生不得修改窗体文件中已经存在的控件和程序，最后将程序按原文件名存盘。
[题目提供的源程序]
Dim flag As Integer
Private Sub Cmd_Click(Index As Integer)
If Index= (1) Then
End
Else
’考生文件夹下的图片文件
Image1.Picture=LoadPicture("yellow.ico")
flag=Index
Timer1.Enabled= (2)
End If
End Sub
Private Sub Timer1_Timer()
Select Case (3)
Case 0
Image1.Picture=LoadPicture("green.ico")
Timer1.Enabled=False
Case 1
Image1.Picture=LoadPicture("red.ico")
Timer1.Enabled=False
End Select
End Sub