An example of a grad-school level question?
To compare the hardness of different minerals, it would be BEST to find

An example of a grad-school level question?
Which of the following is the major cause of tides?

An example of a grad-school level question?
As the population in an area grows, water usage increases. Eventually, water usage can outstrip the supply. Which measure to increase the amount of available water demonstrates the best stewardship of the resource?

An example of a grad-school level question?
Which provides the oldest evidence for oxygen accumulation in Earth's atmosphere?