prepositions - "along the river" vs "alongside the river"?

In a question on "along" vs "alongside", I have seen this question and the correct answer on the web site makes me confused.

http://www.learnex.in/difference-between-along-and-alongside-free-english-lesson/

The question is: I enjoyed my walk _________________ the river.

a) along b) alongside

I selected "...along the river" as the correct answer, but on the web site it is shown as wrong and the correct answer is "alongside".

How can that be? I still do not agree, because I have studied the difference between the two. "along" is just the right preposition for thin and long places like road, path, street, beach etc".

So, why would the correct answer be "alongside" which simply means "next to"? Do you not think "along" seems to be better fitting?

Regards,

Note: I would be more than gratefull, if somebody show it to me in a video, like "a walk along the river" and "a walk alongside the river". Maybe I can understand the how "a walk along the river" and "a walk alongside the river" would take place.