THE PASSIVE VOICE: AGENT vs OBJECT FOCUS

Agent vs. Object Focus (From your teacher) 

PROJECT FOCUS
In science, business, and other collaborative fields, writers place the project in the subject position and the workers in a by phrase or a list at the end of the writing. This effectively shifts the focus to the interesting, important content and avoids repetition of the agent(s).
The human genome has been mapped by scientists.
Three billion base-pairs of DNA were determined. (by scientists who are too many to list)
This knowledge is expected to lead to better treatments for diseases like cancer and diabetes.  (by an unknown future research team)
The discovery must be linked to responsible use.  (by everyone)

AGENT FOCUS
Reporting information with active sentences is preferred whenever possible because the inclusion of the agent(s) is more personal; and the use of active verbs is more dynamic. However, sometimes there are too many agents to list and the collective work is the actual focus.
Scientists have mapped the human genome.
Scientists have determined three billion base-pairs of DNA
Scientists expect this knowledge to lead to better treatments for diseases like cancer and diabetes.
Everyone must link this discover to responsible use.

 You will find some interesting exercises at:

  http://grammar-quizzes.com/passive1a.html