It's 'in those days'.. specifying those days as in the past.
Answer:
Focus
Explanation:
To specialize in something means to bring the focus of your work and interest in that. <u>The person who is professionally specialized for some field of research is focusing their work, analysis, experiments, and career on one subject, trying to perfect it completely and to find the best possible results of it. </u><u>They are focusing to become the best they can in some skill or field, with the idea that they can become an expert. </u>
For example, someone might be a doctor, but they are specialized in dermatology. That means they have basic medical knowledge and academic background in all medical fields. Yet the focus of their work and what they are really good at are the skin problems and illnesses.
