Not necessary, because the author did not conclude that either yoga or stretching classes were the MOST effective at reducing back pain.

Necessary, because stretching classes and yoga had “equal reductions in chronic lower back pain.” So if (B) were not true — if taking stretching classes does NOT significantly reduce chronic lower back pain — then that implies yoga also does not have that effect. In which case we’d have no reason to conclude that doctors should be prepared to discuss the merits of yoga.

Not necessary, because whether there are many other treatments available has no bearing on whether doctors should be prepared to discuss the merits of yoga. We know that if an activity greatly reduces chronic lower back pain, doctors should be prepared to discuss it. That rule applies no matter the number of other treatments available.

Not necessary, because even if there have been some studies that have compared yoga and stretching classes before, that doesn’t change the results of the study mentioned in the argument and the kinds of conclusions we can draw from that study.

Not necessary, because whether many doctors currently discuss the merits of stretching classes has no bearing on whether doctors SHOULD be prepared to discuss the merits of stretching or yoga.