Crossing-over is the exchange of genetic material between homologous chromosomes. It results in new combinations of genes on each chromosome. When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other.
Changes in groups of organisms over time produce variations in a population. This relates to both macroevolution and microevolution.
These are evolutions within a small group or a population and across the whole species.
A pathogen is a disease carrying vector that has the ability to spread using a variety of different mechanisms. A pathogen that is a contagion has the ability to spread through touch. An example of such a pathogen would be the one that results in staph infections.
The correct answer among the choices listed above is option D. The connection between the mother and the embryo that provides the embryo with oxygen and nutrients and waste disposal is the placenta. Hope this answers your question. God Bless.