Answer: a. The concentrations of the reactants and products have reached constant values
Explanation:
The reactions which do not go on completion and in which the reactant forms product and the products goes back to the reactants simultaneously are known as equilibrium reactions. For a chemical equilibrium reaction, equilibrium state is achieved when the rate of forward reaction becomes equal to rate of the backward reaction.
Equilibrium state is the state when reactants and products are present but the concentrations does not change with time and are constant.
Equilibrium constant is defined as the ratio of concentration of products to the concentration of reactants each raised to the power their stoichiometric ratios. It is expressed as 
K is the constant of a certain reaction when it is in equilibrium, while Q is the quotient of activities of products and reactants at any stage other than equilibrium of a reaction.
For a equilibrium reaction,

![K_{eq}=\frac{[B]}{[A]}](https://tex.z-dn.net/?f=K_%7Beq%7D%3D%5Cfrac%7B%5BB%5D%7D%7B%5BA%5D%7D)
Thus the correct answer is the concentrations of the reactants and products have reached constant values.
Answer:
A type of an atom which has a different number of neutrons but the same atomic number, therefore making it the same element. This atom would still have the same properties as well. (Ex: Vanadium-51 is an isotope of Vanadium that has 51 neutrons but still has 23 protons, as its atomic number is 23.)
Silicon dioxide is SiO2. Silicon has 4 valence electrons, while each oxygen has 6 valence electrons. This can be shown as
** * *
** O * * Si * * O **
* * **
At points where there is one valence electron, represented by a lone *, the electrons will be 'shared' between the atoms. This will make silicon dioxide appear as
** *--------------- *
** O *--* Si * ------ * O **
*------- * **
, as the lines with no arrows indicate that each electron moves between the atoms, and does not stay with one forever.