Ponds are
a.
typically larger than lakes
b.
able to develop self-contained ecosystems
c.
distinguished by easily observed currents
d.
another name for stream pools

I'd say its B. able to develop self-contained ecosystems. 

Dall of the above all of the choices make sense

This is all you, but i can give you some topics that you could try. if it rained when you want to do something outdoors, it 'limits' that human activity. maybe you want to do an experiment that goes throughout the whole year and you have to have above freezing the entire year. this 'limits' your activity. i thought of those just off of the top of my head. something that gets to me that might be getting to you is you are thinking too hard about what to do. think simply, and if it is too simple, then bump it up just a little bit.