Option (C) is the correct answer.
In the chemical reaction, reactants are the starting elements or compounds which will chemically react and will form the products. Reactants are only present on the left hand side of the reaction and products are present on the right hand side of the reaction.
Reactants present in the reaction is completely used up in the reaction to form products implies no traces of reactants will be present in the product side.
For example:

Here, hydrogen and oxygen are the reactants whereas water is the product (new substance). Hydrogen reacts with oxygen and form water i.e. hydrogen and oxygen is completely used up in the reaction to form new substance i.e. water.
Hence, substances that are used up in a reaction is known as reactants of the reaction.
Answer:
The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion.
Answer:
The main function of the immune system is to provide immunity to the body. Immunity is the body's ability to protect itself from the pathogens it is exposed to. Immunity depends on the body's ability to differentiate between things that belong in the body and those that do not. The immune system's first line of defense includes physical and chemical barriers that are always ready and prepared to defend the body from infection.
The skin acts as a barrier against pathogens. Some structures of the first line of defense include the skin, mouth, eyes, ears, nose, and stomach. The skin produces sweat and oil that work together to keep pathogens from the entering the body. Saliva and tears contain substances the can destroy pathogens. The ears and the stomach help fight pathogens.The ears produce ear wax that prevent pathogens from entering the ears. The stomach produces a liquid the kills pathogens.
Mucus is produce by the nose, lungs, throat, and stomach. Mucus traps pathogens. Mucus also prevent pathogens from getting into deep tissues. Anyway, the very first line of defense against any invasion of the human body is a set of physical barriers between the inside of the body and the outer world.
*my notes from edge2020*
Hope this helps ^-^