I originally chose B) based on the chain of reasoning that JY said (assuming that difficulty meant the inability to stretch). It took a minute for me to understand how D) is correct, but this is how I reasoned with this answer.

This answer choice introduces a difference between the two groups that deviates from an ideal experience where everything is controlled. The answer choice is basically saying that the one group with the habit of jogging could potentially also just be more prone to injuries, revealing an aspect that is uncontrolled and could affect the validity of the argument.

@emann100 literally started doing a happy dance and then stopped abruptly

ouch

mapping it out and kicking ideas up to the domain definitely helps with these question. remembering the group indicators definitely helps me with these question.

@stratford.douglas This helped me understand, but then I got a bit confused when the review suggests that A -m->B and A-m->C yield a valid conclusion and doesn't include all. Using your example would that mean:

Most NBA players are tall. Most NBA players are bad at basketball. Therefore some tall people are bad at basketball?