PT152 S1 Q10 ExplanationWhen a bird flies, powerful forces converge

Answer choices, explained

1. A

   Too Strong: the only1% picked this

   the only structure that is indispensable to

   We know that this ligament is indispensable to bird flight, but it doesn't have to be the only thing that is indispensable. We know that it's the only structure that can balance the forces on the bird's shoulder joints; that's what makes it indispensable. But we don't know that converging forces on shoulder joints are the only technological problem a bird's body must solve. Maybe it needs to have special structures in its ear canals in order to move fluid around so that the bird can balance in any spatial orientation. Maybe it needs a rib cage that is light enough for flight but robust enough for flapping. We have no idea.

   1%
2. B

   Must vs. Can3% picked this

   the reason that a bird’s wings must be kept stable

   The ligament is the reason that a bird's wings can be kept stable during flight. The reason a bird's wings must be kept stable is something we didn't hear about, but it probably relates to aerodynamics and lift and other mechanics of managing air currents.

   3%
3. C

   Too Strong: the sole connection2% picked this

   the sole connection between the wing and the

   We know that the ligament is a vital / necessary connection, but we don't know if it is the only thing connecting the wing and the shoulder joint.

   2%
4. D

   Out of Scope: source of forces7% picked this

   the source of the powerful forces that converge on the

   It doesn't make a lot of sense to say "powerful forces are converging on the White House. There's only one person who can balance them: Jack Ryan", and then say "by the way, Jack Ryan also is the powerful forces converging on the White House." I was assuming that the powerful forces converging on the shoulder joints were a combination of wind and the heat given off by wing-flapping. But we have no information about the source of the forces -- we just know that the ligament is the thing that manages these forces.

   7%
5. E

   Correct87% picked this

   the means by which a bird stabilizes its wings

   Why this is right

   In our prephrasing, we said the ligament is "the only thing inside a bird that is capable of stabilizing its wings to enable flight". It's a pretty close meaning match to say the ligament is the means by which a bird stabilizes its wings. We know that a bird couldn't stabilize its wings without balancing forces, and that the only thing that can balance the forces is this ligament. Hence, this ligament is the means by which the bird stabilizes.

   Skill tested: Most Supported · how this choice captures the argument's function is the move to repeat next time.

   87%