active field period的用法和样例：
- After entry into the new period,the study of Cantonese has become a very active field of research and the study of its word order is a hot spot.
- In recent years, the research of problem posing is a very active field.
- The Activity field appears in the merge area.
- The main factors include laser energy, electricfield distribution along the laser beam, electric field period, air condtion.