The solar nebula theory
select one:
a. explains how to determine the distances to other stars
b. is a scientific idea about the origin of our solar system
c. provides scientific proof that pluto is a comet
d. is an idea on how to stop global warming

b. is a scientific idea about the origin of our Solar System

Explanation:

The nebular hypothesis provides the fact that the sun, stars, and all planets were formed from the same amount of dust and gas that shaped the earth some 4.5 billion years ago. According to the nebular hypothesis, a star forms a dense and massive cloud of molecular hydrogen and then due to gravitationally unstable matter rotates, collapses and then forms a star.  This star formation process naturally results in a disk of accelerated concluded by a gaseous tail. The formation of rocky planets was a result of the solar nebular disk model with the movement of the protoplanetary disks.

D.haka is the first answer and a.canberra is the second answer

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.