What is one example of biological variation that Darwin observed in his travels?

Consider the model of the oxygen cycle on earth. the majority of earth's oxygen reservoirs are found in the a) atmosphere. b) biosphere. c) hydrosphere. d) lithosphere.

Control of the body is accomplished by which of the following body systems? nervous system and circulatory system endocrine and repertory system circulatory and respiratory systems nervous system and endocrine systems

The structure of a dna molecule is best described as a. one long strand of nucleic acids b. two strands of dna nucleotides joined by hydrogen bonds c. two strands of amino acids joined by hydrogen bonds d. one long strand of amino acids