What exactly does it mean that they are "keeping" their milk? Are they protecting it?
The cats would not "keep" the milk. They either finish it off or leave. Or they guard it, if they feel a threat from other cats. Duo marked "guard" as wrong.
Totally agree. I think Duolingo should change this to שומר אל, which is what we all want to say here. The probability of that increases if we all flag (report) it.