ANSWER: d. all of the above
EXPLANATION: For humans to exist, there are number of factors needed to fulfill. These include food availability because without food people would die out of famine throughout the world. This is the basic necessity of a living animal to survive. Freshwater is another factor which is considered to be basic in order to survive. Living animals such as humans need freshwater for drinking. There are many other living animals which are capable of living in salty water from the sea but humans are excluded from the list. Living space availability is another important factor which are often neglected to be considered. After a certain point, if there are shortage of living space then it would be impossible for people to survive as that would be suffocating for everyone. To avoid this, humans are looking for alternative planets to live and are continuing space explorations on a regular basis. So, these are the reasons why all the factors are necessary for humans.
