Because we study the present in this unit, and they choose a meaning where she builds cities regularly. (Maybe it's a game or something).
You can say here "is building" if the simple present looks a bit disturbing.
Condit is the present: simple present or progressive present.
Both is correct. But since languages seldomly allow precise translations of words, a word in one language will probably have several meanings in another, varying depending on context. This course is still in Beta, so if you think your answer should be accepted, report it. The admins are only humans as well and will not have thought of every single possible translation to each sentence right off the bat. ;)
look, duolingo just wants you to understand the gramatical structure and the new words presented. if you think you can handle more than what you are thaught, you can allways download an app to boost your vocabulary. plus, sometimes its quite funny when you are presented with something like "i am sorry, i am an apple", it really breaks the ice
btw, you confuse overpopulation with overcrowding, but this subject isnt even fit for a duolingo forum