Long before man was a dominate species on earth, there were shifts in earth's climate. according to the data recorded in the graph seen here, we would assume that a major climatic shift on earth, from tropical to frigid, occurred about 34 million years ago. we know there have been several ice ages on earth. if such climatic shifts are naturally occurring and not the result of some catastrophic event like an asteroid impact, how can we explain them?

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.