Answer:
A. to enter a cell at a specific receptor site.
Microscopic, single-celled organisms that have a complex structure similar to animals and plants are known as protoctists.
C. Carbon
Mercury is dangerous to humans
Carbon is the main element in organic compounds, so carbon is essential to life on Earth.
Answer:
The antibodies in the allergic response to dog fur do not fit on the antigens present on a peanut
It is useful for the walls of capillaries to consist of a single layer of thin, flattened cells in order to easily diffuse oxygen and nutrients easily from the blood and into the capillaries. Not only that, but this also makes it more efficient for carbon dioxide and other waste molecules to diffuse from the capillaries and into the blood. If capillaries were thicker than a cell, then it would be more difficult for materials to diffuse through the thick layer of cells.