A. making an observation. 
Explanation:
When scientists use one of their five sense to gather information, they are simply making an observation. 
Most physical properties of matter are studied using sense of taste, of feel, sight, hearing and smell. 
- Observation is the act of making a measurement or studying. 
 - It helps to investigate particular phenomenon in nature. 
 - Most observations made with our senses are usually qualitative in nature. 
 - They cannot be assigned values like quantitative ones. 
 
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Answer:
![K_a=\frac{[H_3O^+][HCO_3^-]}{[H_2CO_3]}](https://tex.z-dn.net/?f=K_a%3D%5Cfrac%7B%5BH_3O%5E%2B%5D%5BHCO_3%5E-%5D%7D%7B%5BH_2CO_3%5D%7D)
Explanation:
Several rules should be followed to write any equilibrium expression properly. In the context of this problem, we're dealing with an aqueous equilibrium:
- an equilibrium constant is, first of all, a fraction;
 - in the numerator of the fraction, we have a product of the concentrations of our products (right-hand side of the equation);
 - in the denominator of the fraction, we have a product of the concentrations of our reactants (left-hand side o the equation);
 - each concentration should be raised to the power of the coefficient in the balanced chemical equation;
 - only aqueous species and gases are included in the equilibrium constant, solids and liquids are omitted.
 
Following the guidelines, we will omit liquid water and we will include all the other species in the constant. Each coefficient in the balanced equation is '1', so no powers required. Multiply the concentrations of the two products and divide by the concentration of carbonic acid:
![K_a=\frac{[H_3O^+][HCO_3^-]}{[H_2CO_3]}](https://tex.z-dn.net/?f=K_a%3D%5Cfrac%7B%5BH_3O%5E%2B%5D%5BHCO_3%5E-%5D%7D%7B%5BH_2CO_3%5D%7D)
 
        
             
        
        
        
Your Answer Will Be Intensive Property