Fragment的有关问题，Error inflating class fragment

Fragment的问题，Error inflating class fragment
就几行很简单的代码：

MainActivity.java

package com.plusjun.action;

import android.os.Bundle;
import android.app.Activity;
import android.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		super.onCreateOptionsMenu(menu);
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return false;
	}

	@Override
	public boolean onOptionsItemSelected(MenuItem item) {
		return super.onOptionsItemSelected(item);
	}
}

class MyFragment extends Fragment{
	
	@Override
	public View onCreateView(LayoutInflater inflater, ViewGroup container,
			Bundle savedInstanceState) {
		return inflater.inflate(R.layout.a,container,false);
	}
}

activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

    <fragment
        android:id="@+id/fragment1"
        android:name="com.plusjun.action.MyFragment"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</LinearLayout>

a.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Button
        android:id="@+id/bt1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="null" />

</LinearLayout>

报错日志如下：


搞了我三天了。求解答
------解决思路----------------------
support v4 lib加的对么