Photosynthetic cyanobacteria caused an increase in atmospheric levels of a) ozone b) water c) oxygen d) carbon dioxide.

C...the key is that they are photosynthetic...so they produce oxygen (like plants)!

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: