A keystone species is one . A. that makes up a very large proportion of total community biomass B. that feeds on a very large fraction of all available prey species C. that is fed on by a very large fraction of all predators in its community D. whose feeding activities have a disproportionate effect on the structure of its community E. that occupies the lowest level (the base) of the food web

these muscles maintain the blood pressure in the vessels. the smooth muscles are innervated by nerves of the autonomic nervous system. when the muscles contract, they cause vasoconstriction of the vessels and increase blood pressure. when the muscles relax, they enable vasodilation of the   blood vessels hence reduce blood pressure.  

answer///africa and asia;

the cell is the smallest structural and functional unit of living organisms, which can exist on its own. therefore, it is sometimes called the building block of life. some organisms, such as bacteria or yeast, are unicellular—consisting only of a single cell—while others, for instance, mammalians, are multicellular.