"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.

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.

This may be tricky at times cause they are so close in meaning. The main point about the "next to" definition is to understand that in most cases "along" means "forming a line (that has a start point and an end point)" whereas "alongside" simply means "next to; beside" - it doesn't imply that something forms a straight line. - Alongside vs Along

There however is a bug in this difference since "along" can also mean "going lengthwise":

• We were having a romantic walk along the river.
• We were having a romantic walk alongside the river.

This also concerns other definitions which eventually are alike.

Eventually both the choices are correct. However, people most often say "along the bank of the river" (not alongside) or "along the river coast" (not alongside).

• along doesn't mean "from one part ... to another"; it does not entail two points, if that is what you meant. It refers to moving roughly parallel to something which has length, or following that length. Jan 19, 2018 at 12:49
• @Tᴚoɯɐuo I took dictionary definitions which are obviously correct. Jan 19, 2018 at 12:57
• I'm afraid you've misunderstood them. Care to share them? Starting point and ending point are not essential to the idea of along. Jan 19, 2018 at 13:00
• And even Cambridge, trying to be helpful, gets it wrong: "Along also means at a particular point: My office is the third door along the hallway on the left.". along does not refer to the point but to the line on which the point is found. Jan 19, 2018 at 13:06
• @Tᴚoɯɐuo actually that's what my fav. Cambridge is actually saying dictionary.cambridge.org/dictionary/english/along Jan 19, 2018 at 13:08