Natural selection could not occur without:
a. genetic variation in species
b. environmental changes
c. competition for unlimited resources
d. gradual warming of the earth

the answer for this question would be d. glaciers

in the year 2850, humans successfully colonized mars. the martian modules that were constructed could hold only a small population of people. it is now a century later, and the population on mars has grown to 600 people. of the current mars population, 45 people express the recessive phenotype for phenylketon- uria (pku) syndrome (pp genotype).

explanation:

i won't lie i don't know so go online and ask google thats what i do

they had the law tat factories that produces smoke cannot be built near residential buildings.

building of proper sewage system was done to ensure that the waste is carried away from the humans so that they do not get diseases from it.

also they built water systems to provide water in those areas which are drought prone.