Natural selection work's at the individual level, selecting those individuals who will more likely be able to contribute their genes to the gene pool of future generations. Therefore, an organism's is determined mainly by how many offspring they produce throughout their lifetime.

littering streams           is the answer

biology is the study of life. it is the study of living organism.

meiosis is the first to occur after the nuclei of opposite of opposite mating types fuse

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.