Dear, Editors
I was making a quiz on rivers and wanted to ask if I should include the river system or just the river such as if I wantd to ask which one is longer, the Mississippi or the Congo, if I included the Missouri, then, that would change the answer. I don't know what to do.
Please help.

What you need to do is specify when you write your question, so that the player will know what you are counting in your question.

For example,

Including all of their respective river systems, which of these rivers is the longest?

Versus:

Not including any of their river systems, which of these rivers is the longest?