In other words, (A) says that anyone who was already unemployed does not reduce their driving. Charles is talking about decreasing pollution from cars, but (A) tells us that a certain group's car use would not decrease. This is not necessary: it goes in the opposite direction from Charles' claims.

(B) strengthens Charles' argument, but is not necessary because it goes further than a truly necessary assumption would.

We can test this by negating (B), which would tell us that commuters cause a minority of pollution from cars. But even a small decrease in pollution from commuters could still reduce pollution overall. The conclusion still follows, meaning (B) isn't necessary.

Like (B), (C) might strengthen by increasing the environmental impact of commuters, but that doesn't make it necessary. An assumption is only necessary if the argument would fall apart without it.

To check this, we can use the negation test. Doing so, we see that even if only a minority of commuters drive, decreasing their driving time could still decrease pollution. So the argument stays together, meaning (C) isn't necessary.

In other words, decreasing commuting truly does decrease car use overall. This must be true in order for the conclusion to follow, because it protects the causal chain. If we negated (D), and commuters’ reductions in driving were outweighed by increases in driving for other reasons, then the causal chain would be broken. That would destroy the argument, so we know (D) is needed to get to Charles' conclusion.

Like (B) and (C), this may strengthen, but isn't necessary. Even if we negated (E), fewer commuters driving could cause an overall decrease in car pollution regardless of whether that decrease was big or small. Since the argument stays apart even if (E) is negated, (E) is not necessary.