Why are rocks an imperfect record of the history of the planet?
a. rocks can be easily misplaced
b. earth is dynamic and the record erodes
c. rocks are inherently untrustworthy
d. it is impossible to know the age of a rock
e. all rocks are the same age
what is a surface of erosion or nondeposition?
a. a conformity
b. a fossil bed
c. if moss covered a nice place for a nap
d. a hiatus
e. an unconformity

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.

He introduced the separation of religion and government 

Ido believe it is b 

Granite is a hard stone and lasts for a long time in both modern and ancient buildings. as proof of its durability, granite is often used in gravestones and memorials, intended to last for many years.