I am pretty sure that the actual answer hides in the question itself, therefore I'd choose the second option. According to the phrase "he more successful of the two", it definitely should be B. Competition.
The different barriers that you have to go through are the different organs of the immune system. These are the lymphoid organs, thymus, and bone marrow. The secondary wall that you have to go through are the lymphatic tissues which include the lymph nodes, adenoids, skin, liver, tonsils, spleen, and the lymph vessels.
At all times, these systems work hand-in-hand so that the body can protect itself in order for it to survive
Answer: Organism in this domain can be defined as living things that has an organized structure, and responds to stimuli, and has ability to reproduce, grow, and maintain a state of equilibrium. Organisms are of two types which are unicellular and multi cellular organisms.
Explanation: unicellular organisms are organisms that are made up of only one cell which carries out all the functions needed by the oragnism. Multicellular organisms are organisms that are made up of more than one cell. These organisms uses many different cells to function.
Unicellular organisms are often found in extreme environments such as hot springs, polar ice and frozen tundra.
Unicellular organisms have cell that contain a nucleus.
