Let's reanalyze each option with this in mind:
(A) Pitchers are invariably under a great deal of pressure when throwing to first base.
This might contribute to errors but does not specifically explain why pitchers have a higher error rate compared to other players.
(B) First baseman are aware of the rate of throwing errors for each position.
This does not explain the discrepancy; it only indicates that first basemen are knowledgeable about error rates.
(C) As a rule, pitchers are unable to throw a baseball in such a way that it travels in a straight line.
If pitchers generally cannot throw in a straight line, it directly affects their ability to make accurate throws to first base. This explains why they have a higher error rate despite their general pitching accuracy.
(D) Pitchers are accustomed to throwing from an elevated mound, but most of their throws to first base are from flat ground.
This could be a factor, but it might not fully explain why the error rate is higher. Pitchers are professionals and can adjust to different conditions to some extent.
(E) First baseman almost never have to catch a ball traveling faster than 80 mph and the average major-league fastball is 92 mph.
This concerns the speed of the throw rather than the accuracy, which doesn't directly address the discrepancy.
Upon re-evaluation, option (C) indeed provides a direct explanation for why pitchers might have a higher error rate when throwing to first base: their inability to consistently throw in a straight line. This fundamental issue with their throwing mechanics would naturally lead to more errors.
So, the correct answer is:
(C) As a rule, pitchers are unable to throw a baseball in such a way that it travels in a straight line.