Explanation

When we are dealing with speeds, we generally are dealing with three pieces of information, speed, distance, and time, and two pieces of information will be sufficient to determine the third and solve completely. This question has two cars, so we could think of it as having six variables, but since it's a question of one car catching up with another (actually, falling behind), it's really a combined rate question. We'll see this in the data statements, which we'll look at separately first.

Statement (1) allows us to conclude that Car C is falling behind Car D at a speed of 10 miles per hour. That means the gap will increase from 10 miles to 20 miles in one hour. We have answered the question definitively, so Statement (1) is sufficient.

Statement (2) appears to give similar information. It tells us that the gap increased from 5 miles to 10 miles in half an hour, it is increasing at a rate of . Again, that means the gap will increase from 10 miles to 20 miles in one hour. Again, we have answered the question definitively, so Statement (2) is sufficient.

The correct answer is (D).