Scientists have been MAKING / DOING / TAKING / LOOKING INTO. research to discover the cause of the disease.
DOING
The project is quite easy so you won't need to HAVE / SPEND / PASS / DEVOTE very much time to doing it.
DEVOTE ....TO
Igor felt very tired because he had PUT / DEVOTED / GIVEN / SPENT a lot of time and energy to the project.
SPENT (time)