If a doctor knows that a cat has an eye disease but is unsure of the cause, the doctor chooses the drug
a. that s costly.
b. based on the site of action.
c. based on its primary effect.
d. based on the desires of the client.

The correct option is C.
In the situation described above, the doctor has to treat the cat based on the symptoms that the cat is showing, that is, the doctor has to treat the effects of the disease that is been shown by the cat.If the cat's eye is red and puffy for instance, the doctor has to prescribe a drug that will remove t he redness and reduce the puffiness. But if the pathogen that cause the eye disease is known, then the doctor will recommend a drug that will kill the pathogen.  

The answer i think would be diet coke