Question:
Professors do a lot for students, but they are rarely thankful.

they
Answer:
students
Question:
The older male cat is bullying the new younger male cat so that he can assert his dominance.

he
Answer:
The older male cat
Question:
The mechanic charged the man one hundred dollars although he only had eighty dollars.

he
Answer:
the man
Question:
The Drill Sergeants trained the troops aggressively because they need to be trained well to survive in war.

they
Answer:
the troops