In the elementary reaction, $2A + B \to {A_2}B,$ the reactant A will disappear at:
A. half the rate that B will decrease
B. twice the rate that B will decrease
C. the same rate that B will decrease
D. the same rate that ${A_2}B$ will form

Hint: We must know that a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. A chemical mechanism describes in detail exactly what takes place at each stage of an overall chemical reaction.

Complete answer:
By referring to the Rate Law of a reaction can be predicted only from experimental data. Once rate law is derived then one can predict order of the reaction. Also we can predict the rate of disappearance of some reactant with the help of the rate law. The rate of a chemical reaction is the change in concentration over the change in time and is a metric of the "speed" at which a chemical reaction occurs and can be defined in terms of two observables:
The Rate of Disappearance of Reactant is written as,
\[ - \dfrac{{\Delta \left[ {{\text{Reactants}}} \right]}}{{\Delta t}}\]
Note this is negative because it measures the rate of disappearance of the reactants.
The Rate of Formation of Product is written as,
$\dfrac{{\Delta [{\text{Products}}]}}{{\Delta t}}$
This is the rate at which the products are formed. They both are linked via the balanced chemical reactions and can both be used to measure the reaction rate.
 According to the given equation, the rate of disappearance of reactant A is twice the rate of disappearance of reactant B.
$ \Rightarrow \dfrac{{ - 1}}{2}\dfrac{{d[A]}}{{dt}} = \dfrac{{ - d[B]}}{{dt}} = \dfrac{{d[{A_2}B]}}{{dt}}$
So, we know that the reactant A will disappear at twice the rate that B will decrease.
Therefore, the option B is correct.

Note:
Now we discuss about the some of the features that affect the rates of the reaction as,
-The concentrations of the reactants: Most chemical reactions proceed faster if the concentration of one or more of the reactants is increased.
-The temperature at which a reaction occurs: The rates of chemical reactions increase as temperature is increased.
-The presence of a catalyst: A catalyst is a substance added to a reaction to increase its rate without being consumed in the reaction. The most common catalyst is enzymes.
-The surface area of solid or liquid reactants or catalysts: Reactions that involve solids often proceed faster as the surface area of the solid is increased. For example, a crushed aspirin will enter the bloodstream quicker.