The factor(s) that determine the duration of a twitch in various types of fibers is the speed of the a. removal of ca2+ ions from the sarcoplasm only. b. hydrolysis of atp only. c. movement of na+ across their membranes only. d. removal of ca2+ ions from the sarcoplasm and hydrolysis of atp. e. removal of ca2+ ions from the sarcoplasm and hydrolysis of atp and movement of na+ across their membranes.

During crossing-over, a. genetic material is exchanged between nonsister chromatids, resulting in new combinations of alleles. b. nonsister chromatids from each homologous chromosome of a tetrad are exchanged, resulting in new combinations of alleles. c. one homologous chromosome of a tetrad is exchanged with another tetrad, resulting in new combinations of alleles. d. sister chromatids from each homologous chromosome of a tetrad are exchanged, resulting in new combinations of alleles. e. genetic material is exchanged between sister chromatids, resulting in new combinations of alleles.