Self-study
Support If a person chooses to walk rather than drive, there is one less vehicle emitting pollution into the air than there would be otherwise. █████████ ██ ██████ █████ ████ ████████ ██ ██ ████████ ███ ████ ██ ██ ███ ████ █████████ ████ ██ ███████ ████████
Argument Breakdown
The author concludes that if people would walk whenever feasible, then that would greatly reduce pollution. Why? Because if someone walks instead of driving, then that avoids one car's worth of emissions. In other words, because walking rather than driving reduces pollution, walking whenever feasible would reduce pollution.
Objective: Identify a Necessary Assumption
Notice that the author treats two ideas as interchangeable: walking whenever feasible and walking instead of driving. This means the author must assume that it is sometimes feasible for people to walk when they currently drive. Not only that, but this must be common enough to have the potential to significantly reduce car pollution.
We know this assumption is necessary because the argument falls apart when we negate it. If people already only drove when it wasn't feasible to walk, then walking whenever feasible couldn't make any difference.
9.
Which one of the following, ██ █████ ████ ███████████ ███ █████████
a
If automobile passengers ███ █████ █████ ████ ███████ ██ █████ █████ ████ ███ ██ █████ ████████ ██ ███ ████ ██ █ ███████
The argument is only concerned with the choices of people who drive, so the choices of passengers who never drive aren't relevant.
b
Nonmoving running vehicles, ██ ████████ ████ ████ ██ ████ █████████ ███ ██████ ██ ██████ █████████ ███ ███ ███████ ██████████ ███ ███ ████ █████████ ███████ ████████ █████ ████
In other words, idling cars pollute less than moving cars, but more congestion (i.e. more cars on the road) means more idling cars. (B) gives us an additional benefit to walking: not only directly reducing the number of cars, but also reducing congestion so that other cars spend less time idling and emitting pollution.
c
Since different vehicles ███ ███████ ██ █████████ ██████ ██ ██ ████████ ███ ███ ██████ ███ █████ ██ ████ █ ███████ ████████████ ██ █████████ ██████████ ████ ███████ ██████ ███ ██████
(C) is an extremely reasonable claim, but because we don't know what types of cars are driven by people who could feasibly walk, it doesn't actually make any difference to the argument. We can't assume that more-polluting cars are distributed in any particular way.
If drivers who could feasibly walk were also likely to drive more-polluting cars, then (C) would be a great answer. But without that information, (C) ends up being a trap.
d
On average, buses ███████ ████ ████ ████ ███ ███ █████ ███████ █████ ████ ██████████ ████ ████ ███
The author is only concerned with people who drive but could choose to walk. Like the car passengers mentioned in (A), bus passengers aren't relevant to the argument.
e
Those who previously ████ ██ ██████████ ██ █ ███████ █████ ██████ ███████ ██ ████ ███████ ██ █████ █████ ██████████ ██████ ██ ██████
(E) is a slightly weakening answer, because it identifies a potential disadvantage of people choosing to walk. Passengers who take up driving themselves would offset the reduction of pollution.