That's true; Turkish puts nouns in the singular after numbers and most other counting expressions: birkaç saniye "a few seconds", beş saniye "five seconds".
Also, general objects are usually in the singular, according to https://www.duolingo.com/comment/7736911 - so something like "he counts seconds" might be "saniye sayıyor".
But if "seconds" stands by itself (as in this exercise), then only "saniyeler" works, I think.
Good morning Kirsten
I am only quoting a verse of a song by - Sezen Aksu
"Çiçeklerim dökülür her mevsim." - The flowers no longer in bloom fall to the ground & are replaced with other flowers as the flower buds open & bloom. Each season of course - "mevsim."
"Gonca yüklü" - Flower bud that is about to open & reveal a beautiful flower - Like you.
I hope this explains your "?"