Because it really is optional! Same with "δυο". However, the pronunciation changes: when the accent is missing, the two vowels are said in one 'beat', not two. It's like other words, like ''παιδιά', "κρεβάτια", the "ια" is treated as a single syllable. They are pretty much false diphthongs, search for "καταχρηστικοί δίφθογγοι" here and here.
Ε' Δημοτικού is the 10-11 y.o. (it's the fifth year, primary school starts at 6 years of age). I don't have the answers as I don't have the book! I'm surprised the publishing house have uploaded even this part (it's the first 30 pages out of ~250). You can post your questions/answers to an exercise on my stream and I will try to answer them to the best of my abilities :)
@D_. You wrote: "You can post your questions/answers to an exercise on my stream and I will try to answer them to the best of my abilities " Can you explain me "stream" what is it?
Συγγνώμη αλλά δεν καταλαβαίνω στην παραπάνω φράση τι σημαίνει το "stream". Μπορεί ο κάθε χρήστης του Duolingo να το χρησιμοποιήσει