"My cat likes these balls."
Translation:Kucingku suka bola-bola ini.
"Bola" leaves it open as to whether it's one or many. "Bola-bola" specifies it's plural. On the desktop version of Duolingo, there are lesson notes you can read before you start the lesson - so very useful! I gather from Googling it that they are gradually adding notes to the mobile app, but they're not there yet for Indonesian.