Final answer:
The horizontal component of a ball's velocity will decrease as it travels through the air due to air resistance.
Step-by-step explanation:
When a ball is thrown horizontally and experiences air resistance, the horizontal component of its velocity will change. Despite gravity acting only in the vertical direction, air resistance acts against the direction of motion, which here is horizontal. Therefore, the horizontal component of the ball's velocity will decrease over time due to the opposing force of air resistance. If there were no air resistance, such as in a vacuum, the horizontal velocity would remain constant as only gravity would act upon the object, influencing only its vertical motion.