R cannot be resolved to a variable 啥情况啊

补充，AndroidManifest.xml文件如下：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="zz.Activity"
    android:versionCode="1"
    android:versionName="1.0" >    <uses-sdk android:minSdkVersion="8" />    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:name=".Activity01"
            android:label="@string/app_name" >
            
        </activity>
        <activity
            android:name=".Activity02"
            android:label="@string/ff" >
            
        </activity>
        <activity
            android:name=".A01"
            android:label="@string/a01" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
            
        </activity>
        <activity
            android:name=".A02"
            android:label="@string/b02" >
            
        </activity>
    </application></manifest>