B) Oxygen may not have been able to accumulate in the atmosphere
A frog's lungs don't work that well, but since it's skin is so thin, oxygen and carbon dioxide can pass trough it.
Answer:
A hypothesis is usually written in the form of an if/then statement, according to the University of California. This statement gives a possibility (if) and explains what may happen because of the possibility (then). The statement could also include "may."