Language Point 7

Psychologist Arthur F. Kramer tested volunteers working on arithmetic problems (Para. 7)

Meaning: Psychologist Arthur F. Kramer tested willing persons who were trying to solve arithmetic problems

volunteer: n. person who offers to do something without being forced

This is a huge project, but we have a lot of volunteers for the most difficult work.
这是一个大项目，但我们拥有许多志愿者来承担其中最困难的工作。

After the dancing party, quite a lot of young men stayed behind as volunteers to clean the place.
舞会之后，许多年轻人自愿留下来打扫场所。

work on (upon): have (something) as the subject of thought or effort

The scientists are still working on inventing new methods of reaching outer space.
科学家仍在努力探索进入外层空间的新方法。

Is Tom still working on the new book that he promised?
汤姆还在写那本他答应写的书吗？