We should have included it and I've just done that.
However, here is something important to know. The Drop Down Hints, just as a dictionary, may have many definitions but not each one is correct in each sentence. Duolingo has programmed them to keep the word/phrase that fits each sentence at the top. So when choosing your answer ist's safest to choose the top word/phrase.
This time, of course, the second word is also correct.
This is not a big deal....I noted that when I wrote my answer, the word "kilometer" was underlined in red - indicating a misspelling. I noticed this on several other phrases as well. After right-clicking to find out what was wrong, it appears that the spell-checker wanted "kilometres". (But my answer was accepted as correct.) Is this in my spell-checker, or Duolingo's?
In American English we use: "kilometer" in British English we use "kilometre" both are correct. Here on Duo we use American English but we always accept British too. That's why it says "typo" but it never says it's wrong.
All answers on Duolingo are graded by a computer program so it sees the second spelling as a typo but not as wrong.