Many scientists have published, and continue to publish, peer-reviewed journal articles describing their research on the link between nitrate concentrations in drinking water and infant methemoglobinemia. However, there is no scientific consensus in this science: Some studies have found strong links between nitrates and methemoglobinemia, whereas other studies have found no link between nitrates and methemoglobinemia. This discrepancy may be cl variables in measuring nitrates in well water; too many variables is a common in scientific studies. For example, private well water in rural areas also contains high

frontier; limitation

Explanation:

In science, a “frontier” research refers to a type of investigation at the limits of current knowledge, raising concerns about the existing methodology and often accompanied with a speculative hypothesis. In general, frontier research investigates a particular question/issue which is still controversial in the scientific community, thereby challenging the established paradigm. On the other hand, variables are those factors, traits, or conditions which can be measured and changed in an investigation/study. Many variables in a given study design represent an important limitation because they cause noise to establish an association with a particular outcome (in this case, methemoglobinemia disease). A scientific study involves at least one independent variable (i.e., the variable that is not changed during the study) and one dependent variable (i.e., the variable that is changed).

b. heterotroph

explanation:

d) center of the mid-ocean ridge

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