**Neither a teacher nor a native speaker.**
Sometimes it's not the same to say "to verb" or "verb+ing".
I'm not sure if "explain" is such a case.
Anyway, to me the answer is perfect.
Another reason could be that "took" needs the "to".
It took hours to...
If you say, "The Earth has been a sphere", some people could think some day the Earth will not be a sphere anymore.
So it was a sphere, is a sphere, and will always be a sphere.
-> Simple Present
To me this sounds more like some advice.
If you say "could", then it means it's not possible.
"You could try to eat before sunset, but you'll see it's not possible."
"You could try to fly, but you'll see it's not possible."
I hope I could help you.
Student or Learner