The argument tells us that "A series of great works is the only indicator of greatness."
So if great, then you have a series of great works.
G ==> SGW not SGW ==> not G
"Therefore, to say that an artist is great is just to summarize the quality of his or her known works..." So far this part of the conclusion is fine because it goes along with the premise above. We hit our flaw here: "and the artist's greatness can provide no basis for predicting the quality of the artist's unknown or future works."
We have no evidence in the argument giving us info for "future works." The predictive quality of an artist's greatness is something that we have not yet discussed. So, the flaw in the argument is that there is no evidence for the conclusion of the argument.
Likewise in answer choice (A), we know that "The only way of knowing whether someone has a cold is to observe symptoms." So if one has a cold, you know through observing their symptoms.
C ==> OS not OS ==> not C
The first part of the conclusion: "when a person is said to have a cold, this means only that he or she has displayed the symptoms of a cold," is valid, however, the second portion: "no prediction about the patient's future symptoms is justified," has no support in the argument. Again, there is no evidence backing up the predictive qualities of displaying symptoms and/or having a cold.
Hope that was helpful! Let us know if you have any other questions.
