A
Our atmosphere currently holds in significantly less heat than it did 3.8 billion years ago.
B
The liquid water present 3.8 billion years ago later froze, only to melt again about 2 billion years ago.
C
A significant source of heat other than the sun contributed to the melting of ice sheets approximately 2 billion years ago.
D
Evidence suggests that certain regions of ocean remained frozen until much more recently than 2 billion years ago.
E
When large portions of the globe are ice-covered, more of the sun’s heat is reflected and not absorbed by the earth than when only the poles are ice-covered.
A
Each of many different human hormones can by itself raise blood glucose levels.
B
The reason that many different hormones can each independently raise blood glucose levels is probably a metabolic quirk of the brain.
C
Although most human cells can produce energy from fats and proteins, brain cells can produce energy only from glucose.
D
If blood glucose levels fall too low, then brain cells starve, resulting in loss of consciousness and death.
E
The reason brain cells starve if deprived of glucose is that they can produce energy only from glucose.
Human resources director: While only some recent university graduates consider work environment an important factor in choosing a job, they all consider salary an important factor. Further, whereas the only workers who consider stress level an important factor in choosing a job are a few veteran employees, every recent university graduate considers vacation policy an important factor.
Summary
Every recent university graduate considers salary an important factor in choosing a job. Some recent university graduates consider work environment an important factor. All recent graduates consider vacation policy an important factor. The only workers who consider stress level an important factor are a few veteran employees.
Notable Valid Inferences
Some people who consider work environment an important factor also consider vacation policy as an important factor.
Some people who consider work environment an important factor also consider salary as an important factor.
Some people who consider salary an important factor also consider vacation policy an important factor.
A
All people who consider work environment an important factor in choosing a job also consider salary an important factor.
Could be false. We can never infer an “all” statement between conditions with a sufficient condition in common. We can only infer “some” statements.
B
At least some people who consider work environment an important factor in choosing a job consider vacation policy an important factor as well.
Must be true. As shown below, we can combine claims that some recent graduates consider work environment important and that all recent graduates consider vacation policy important. We can always infer a “some” statement between conditions with a sufficient condition in common. 
C
At least some veteran employees do not consider work environment an important factor in choosing a job.
Could be false. We only know that some veteran employees are the only workers that consider stress level important. We cannot infer anything else about these veteran employees. As shown on our diagram, there are no other conditional statements attached to this condition.
D
All people who consider vacation policy an important factor in choosing a job also consider salary an important factor.
Could be false. We can never infer an “all” statement between conditions with a sufficient condition in common. We can only infer “some” statements.
E
No one for whom salary is an important factor in choosing a job also considers stress level an important factor.
Could be false. We can never infer an “all” statement between conditions with a sufficient condition in common. We can only infer “some” statements.
Some countries in favor of increased international peacekeeping efforts are not small countries.
Some countries in favor of a greater role for the United Nations in regional disputes are not small countries.