Why should bulletproof vests be light?

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 35 to 42
DON'T HANDLE WITH CARE
People always go shopping for the latest and greatest gadgets. Speed, performance, and the latest functions are always key factors before making a purchase. Yet the most important factor of all, which often isn't considered, is a product's durability. After all, if a product breaks easily and can't be repaired, the buyer will have wasted their investment. In other cases, weakly-designed product can cost someone their life. Thankfully, today's inventors and engineers coming up with some incredible ways to ensure that many products can survive the most difficult situations.
One area where durability is absolutely essential is in designing weaponry. In the industry, lapses in quality can lead to death. Designers of military vehicles have recently made great strides in creating some extra tough bulletproof vehicles. Researchers were inspired by the advances made with a similar technology, bulletproof vests. This equipment, which keeps police officers and soldiers protected from gunfire, has been continually upgraded as time has gone on. In order to get tougher, these vests have got lighter, so officers and soldiers can move more freely while working.
Beautiful screens, a fast computer processor, and the latest software are probably the biggest concerns for most people shopping for new smartphones. Yet, since most mobiles will be subject to their fair share of bumps and bruises during their life cycle, durability should also be a major concern. A major breakthrough in recent years was the invention of Gorilla Glass. This technology not only prevents devices from breaking when dropped on hard surfaces, but it also makes them scratch-resistant as well. Gorilla Glass screens are given molten alkaline salt baths that produce a hard, compressed layer at the surface. Before a screen can crack, it will have to penetrate this rugged surface, which takes a large degree of force.
Recently, another new technique called electrospinning was developed that could make future products even tougher to break. The process involves applying high voltage to a polymer to create a strand of nanofiber. Products designed with the material will be able to absorb more energy without breaking. Researchers are aiming to lighten up aircraft and potentially decrease the damage planes suffer in crashes. Hopefully, these advances are just the beginning. Although science has come a long way in helping manufacturers to create more durable products, the next century should surely bring some even more incredible improvements.
Why should bulletproof vests be light?
A. Because most of the bullets are not so strong.
B. Manufacturers need to bring down costs.
C. People wearing them need to move more easily.
D. The weather will be too hot if they are heavy.