How easy or difficult was it to find an example of each category?

Depending on what resources you use, it can be easy or difficult to get examples of each categories

Some easy way can include searching google, wikipedia, encyclopedia.org, etc. These usually give you a broad and large amount of information that you can draw out of, and an over view that allows you to know what the paragraph would be talking about before you use it. 

Difficult ways would be searching through specific websites, such as national geographics, etc, which gives you specific details on the idea (and is more trusted then the others)

Remember too always get facts, not opinions, unless your question asks for others opinions, and to look for what researchers or well-known people within that topic says, and to not get any information from "who-knows-who-typed-it" websites, such as ask.com, etc.

hope this helps

Abay where the river empties into the sea

The answer is b. hope this !