Cells are the most simple level of organization, as tissues contain cells, organs contain tissues and systems contain organs, so R would be your answer.
Answer:The excretory system is a close partner with both the circulatory and endocrine system. The circulatory system connection is obvious. Blood that circulates through the body passes through one of the two kidneys. Urea, uric acid, and water are removed from the blood and most of the water is put back into the system.
Explanation:
The amount of water on the planet is fixed; it neither increases or decreases. Glaciers are sheets of moving ice. This water to form these extensive sheets must come from somewhere. The water comes from the most extensive store on the planet; the oceans. Ice Ages always corresponds to periods of low sea level because much of the ocean water is is land locked as glaciers.
An enzyme is a catalyst which means it changes the speed at which a chemical reaction occurs. It never alters the reaction in any way or changes nay chemical properties, only the speed at which it occurs and is completely absorbed in the meantime. The substrates that are reacting just have their reaction faster.
