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现有用户信息表userinfo,表结构如下:id integer 主键,自增长;uname text 用户名;password text 密码。如果想更新用户名为的密码为321,下面方法正确的是() A. private void update(SQLiteDatabase db){ ... String sql="update from userinfo set password = '321' where uname=''"; db.execSQL(sql); ... } B. private void update(SQLiteDatabase db){ ... String sql="update userinfo set password = '321' where uname=''"; db.execSQL(sql); ... } C. private void delete(SQLiteDatabase db){ ... ContentValues cv = new ContentValues(); cv.put("password","321"); db.update("userinfo",cv,"uname = ?",new String[]{"张三"}) ... } D. private void update(SQLiteDatabase db){ ... String sql="update userinfo password = '321' where uname=''"; db.execSQL(sql); ... }