Lab: $graphLookup / ambiguous

In this homework:
Chapter 3: Core Aggregation - Combining Information
Lab: $graphLookup

Though I now have solved it. I need to say the formulation is ambiguous is my opinion.

And if it was not for the provided hint, I would have first tried a different answer.

It is not clear by reading the text of the problem if the restriction (OneWorld alliance; Canada,Germany,Spain) applies to the departure airport only, or also applies to the airline company handling the route.

And if one removes the line:
{$match:{isValid:true}},
one gets the answer that I was first expecting (also given by a different answer).

Beside this, the lab was an excellent way for me to get my hands dirty with some practical exercise.