The referenced text is not the main conclusion, but a subsidiary conclusion. The author concludes that recent proposals are ill-advised. This is supported by the claim that cell-phones would be more annoying on airplanes than they are on trains and buses. This in turn is supported by the claim that airline passengers are usually packed in tightly and the claim that airline passengers often can't move to another seat.

The author doesn’t disagree that cell-phone usage on planes is more annoying than on trains and buses. In fact, he supports the claim with additional premises and uses the claim to support the main conclusion.

The referenced text **directly** supports the conclusion, and is itself supported by other premises. Why are the proposals a bad idea? Because cell-phone usage is even more annoying on airplanes than on trains and buses.

The referenced text supports the claim that recent proposals are ill-advised, and is also supported by two claims. Why are cell phones more annoying on airplanes than buses? Because airplanes are tightly packed, and you often can’t change seats mid-flight to avoid the annoyance.

The referenced text supports the argument. It gives a reason why recent proposals are a bad idea—cell-phone usage, while already annoying on buses and trains, is more annoying on airplanes.