我在comBox2的下拉菜单中设置1，2，3，4四个选项
而我想在comBox1选择a时，comBox2的下拉菜单中只有1，2两个选项
在comBox1选择b时，comBox2的下拉菜单中只有3,4两个选项
请问各位高手们。这怎么实现啊

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace WindowsApplication21
{
/// <summary>
/// Form1 的摘要说明。
/// </summary>
public class Form1 : System.Windows.Forms.Form
{
private System.Windows.Forms.ComboBox comboBox1;
private System.Windows.Forms.ComboBox comboBox2;
/// <summary>
/// 必需的设计器变量。
/// </summary>
private System.ComponentModel.Container components = null;

public Form1()
{
//
// Windows 窗体设计器支持所必需的
//
InitializeComponent();

//
// TODO: 在 InitializeComponent 调用后添加任何构造函数代码
//
}

/// <summary>
/// 清理所有正在使用的资源。
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Windows 窗体设计器生成的代码
/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.comboBox1 = new System.Windows.Forms.ComboBox();
this.comboBox2 = new System.Windows.Forms.ComboBox();
this.SuspendLayout();
//
// comboBox1
//
this.comboBox1.Items.AddRange(new object[] {
"1",
"2"});
this.comboBox1.Location = new System.Drawing.Point(64, 80);
this.comboBox1.Name = "comboBox1";
this.comboBox1.Size = new System.Drawing.Size(80, 20);
this.comboBox1.TabIndex = 0;
this.comboBox1.SelectedIndexChanged += new System.EventHandler(this.comboBox1_SelectedIndexChanged);
//
// comboBox2
//
this.comboBox2.Items.AddRange(new object[] {
"1",
"2",
"3",
"4"});
this.comboBox2.Location = new System.Drawing.Point(176, 80);
this.comboBox2.Name = "comboBox2";
this.comboBox2.Size = new System.Drawing.Size(88, 20);
this.comboBox2.TabIndex = 1;
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
this.ClientSize = new System.Drawing.Size(352, 193);
this.Controls.Add(this.comboBox2);
this.Controls.Add(this.comboBox1);
this.Name = "Form1";
this.Text = "Form1";
this.ResumeLayout(false);

}
#endregion

/// <summary>
/// 应用程序的主入口点。
/// </summary>
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

private void comboBox1_SelectedIndexChanged(object sender, System.EventArgs e)
{
this.comboBox2.Items.Clear();
if(this.comboBox1.SelectedIndex==0)
{
this.comboBox2.Items.Add("1");
this.comboBox2.Items.Add("2");
}
if(this.comboBox1.SelectedIndex==1)
{
this.comboBox2.Items.Add("3");
this.comboBox2.Items.Add("4");
}
}
}
}