I would probably say that the author used diction to achieve some kind of effect in the first sentence.
A. The teacher stood at the door smiling brightly and welcoming each student to her classroom on the first day of school.
The author's diction, or word choice, in this sentence portrays the teacher as a nice, kind, caring person.
An example of an oxymoron is
( C ) ""rosy gloom"
1) Brain Cancer. ...
2) Increases stress level. ...
3) Weakens immune system. ...
4) Chronic pain. ...
5) Eye/ vision problems. ...
6) Hinders sleep. ...
7) Germs. ...
8) Hampers your thinking process.
Depends on what kind of essay, comment me back and tell what kind.