It is easier to roll a barrel full of coal tar than to pull it because A. the full weight of the barrel comes into play when it is pulled B. the rolling friction is much less than the sliding friction C. more surface area of the barrel is in contact with the road when it is pulled D. coal tar is a liquid and it flows in the barrel when it is rolled

The correct answer is: B. the rolling friction is much less than the sliding friction.

When an object rolls, the only force that opposes its motion is rolling friction. Rolling friction is much less than sliding friction because the contact area between the object and the surface is much smaller. This is because the object rolls on a small contact patch, while when it slides, the entire surface of the object is in contact with the surface.

The other options are incorrect because:

• A. the full weight of the barrel comes into play when it is pulled. This is true, but it does not explain why it is easier to roll the barrel than to pull it.
• C. more surface area of the barrel is in contact with the road when it is pulled. This is also true, but it does not explain why it is easier to roll the barrel than to pull it.
• D. coal tar is a liquid and it flows in the barrel when it is rolled. This is true, but it does not explain why it is easier to roll the barrel than to pull it.