C, be-caused. semi -colons and commas.
A really good one that your school would love is the health risks of smoking tobacco.
Its a really simple topic, easy to research and develop, and theres a lot of risks so you have a lot of options.
Hope this helped. Good luck on your speech!
What book is this for? we need more details to answer the question.
This sentence is a fragment, but grammatically you would modify 'often' with either 'more' or 'less'.
Example: "People visit the Empire State Building more often than any other skyscraper"
Equally correct example: "People visit the Empire State Building less often than any other skyscraper."
The reality would be "more often" because the Empire State Building is very popular.