A compelling optical illusion called the illusion of velocity and size

The given statement explains that the illusion of velocity and size causes objects to appear to be moving more slowly as they become larger. Based on this, it is concluded that a motorist's estimate of the time available for crossing a highway with a small car approaching will be lower than it would be with a large truck approaching. Let's evaluate each option to determine which clarifications would strengthen the conclusion:

(A) The truck's speed is assumed to be lower than the car's.
This option does not strengthen the conclusion. While it introduces a speed comparison between the car and the truck, it does not directly address or clarify the impact of the illusion of velocity and size on the motorist's estimate of time available for crossing the highway.

(B) The truck's speed is assumed to be the same as the car's.
This option strengthens the conclusion. If the truck's speed is assumed to be the same as the car's, it reinforces the notion that the illusion of velocity and size is the primary factor influencing the motorist's estimate of time available. As the larger truck approaches, it would appear to be moving more slowly due to the optical illusion, leading the motorist to perceive less time available for crossing the highway compared to a smaller car.

(C) The truck's speed is assumed to be higher than the car's.
This option weakens the conclusion. If the truck's speed is assumed to be higher than the car's, it contradicts the claim that the illusion of velocity and size makes objects appear to move more slowly as they become larger. In this scenario, the larger truck would appear to be moving faster, which would not support the conclusion that the motorist's estimate of time available is lower with a small car approaching than with a large truck approaching.

(D) The motorist's estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching.
This option weakens the conclusion. If the motorist's estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching, it contradicts the claim that the illusion of velocity and size influences the motorist's perception of time. The conclusion relies on the assumption that the illusion causes the motorist's estimate to be lower when a small car approaches, suggesting that the estimate is less accurate. This option suggests the opposite.

(E) The motorist's estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching.
This option strengthens the conclusion. If the motorist's estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching, it supports the claim that the illusion of velocity and size affects the motorist's perception of time. It reinforces the idea that the illusion makes objects, such as larger trucks, appear to move more slowly, leading the motorist to have a higher estimate of time available when a large truck approaches compared to a small car.

In conclusion, the conclusion provided in the statement would be more properly drawn if it were made clear that the truck's speed is assumed to be the same as the car's (option B). This strengthens the conclusion by reinforcing the notion that the illusion of velocity and size influences the motorist's estimate of time available, as the larger truck would appear to be moving more slowly. The other options either weaken the conclusion or introduce conflicting assumptions.