4. A train moves at a constant speed down a straight track. Which of the following scientific explanations is true? A No forced act on the train as it moves.
B The train moves because no forces are acting against it.
C The forces of the train’s engine are acting against it.
D An unbalanced force keeps the train moving.

Engineers find a new metal that is stronger than steel but much lighter. this material is also significantly cheaper than what is currently used for most aircraft, available in large quantities, and easy to manufacture. the engineers are excited because this new material will lower the costs of buying and operating airplanes for companies. what is probably the best step for the engineers to make next?