public static List<Prop> getValidProps(){
		List<Prop> valid = new ArrayList<Prop>();
		Prop instance = null ;
		
		for( int i = 0 ; i < props.size(); i ++ ){
			instance = props.get(i);
			if( instance.getStatus() == PropStatus.ALIVE ){
				if( (int)(System.currentTimeMillis()/1000) - instance.getStartTime() <= Constant.MAX_PROP_TIME){
					valid.add(instance);
				}else{
					props.remove(instance);
				}
			}
		}
		return valid;
	}