explain why crossing-over is an important source of genetic variation. detailed response please, i really need help with this one

Which mantle is under the crust the upper mantle or the lower mantle i think it's upper let me know if i'm right

Which of the following is true of microbes? a. ninety-nine percent of all microbes are pathogenic.b. gene expression in bacteria is very similar to gene expression in humans, which facilitates the use of bacteria in recombinant biotechnology and gene therapy.c. all bacterial enzymes are harmful to humans and the environment.d. microbes create pollutants and toxins that harm the environment.

Compare the depth of field when focusing with low power and high power. which power has the greater depth of field?