//默認選中第一行
NSIndexPath *firstPath = [NSIndexPath indexPathForRow:0inSection:0];
[self._tableView selectRowAtIndexPath:firstPath animated:YESscrollPosition:UITableViewScrollPositionTop];
cell.selectedBackgroundView=[[[UIImageView alloc]initWithImage:[UIImage imageNamed:@"cell_selected_bg"]]autorelease];