Question 50·50 Super-Hard SAT Grammar Questions·Standard English Conventions

The first sentence says that a continental polar air mass acquires unusually cold, dry properties. The next sentence begins Retaining those properties, which points back to the same cold, dry qualities. Who or what retains them even after crossing a milder region? The air mass does, so the completion has to make the opening phrase describe that air mass.

What noun does the opening phrase attach to? An opening describing phrase describes the main noun right after its comma, not any noun nearby. In the southward advance of the air mass, the main noun is advance; of the air mass tells us what is advancing. An advance is movement, not the body of air that keeps those properties, so this wording attaches the description to the wrong thing.

Do meteorologists or the boundary along the air mass’s leading edge name what retains the properties? Meteorologists track the air mass rather than acquire the unusually cold, dry properties themselves. The main noun boundary names the air mass’s edge, not the air mass itself, so the description doesn’t belong to that subject either.

The choice beginning the air mass can puts the described thing right after the comma. Read it with Retaining those properties: the air mass keeps its cold, dry qualities while moving southward, and the rest says what it can do to surface temperatures and the atmosphere. Choice B.