I think the correct answer would be A. The yolk sac becomes the placenta. It is membranous sac that is attached to the embryo. It provides all the nutrition and the blood cells needed by the embryo. It is also called the umbilical vesicle.
<span>When we are talking about biogeochemical cycles we are talking about the pathways by which a chemical substance moves through both the biosphere and the atmosphere. There are a number of cycles which come back to the starting point and which can be repeated. Moreover, the very term biogeochemical tells us that there are more than one factor involved. We have chemical, biological and geological factors at play. The circulation of chemical nutrients through the biological and physical world are known as biogeochemical cycles.</span>
because while plants are exhaling oxygen humans are inhaling it but when humans exhale carbon plants would inhale it.
