adv.周期性地；定期地
New strains of viruses appear periodically. 新菌系的细菌周期性地产生。
The state of knowledge in this field has been surveyed periodically in review papers. 在一些评论文中对这一领域的动态周期性地进行了评述。
Periodically, someone comes by with a truck to fill them. 有些人定期地用卡车运输食品，装进机器。
不定期地
