Which of the following is true about mutation?

a. Mutation comes from artificial selection.

b. Mutation is always harmful to an organism’s survival.

c. Mutation is always helpful to an organism’s survival.

d. Mutation can come from environmental changes

PLZ HELP ME!

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.