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.

Nature has always provided a stimulus for inventive minds. Early flying machines clearly were an attempt to emulate the freedom of birds. Architects and engineers have often consciously modeled buildings on forms found in nature. A more recent example of the inspiration given by nature is the invention of Velcro®. The inventor of this now common fastening device noticed that small burrs attached to his dog's coat grasped the hairs by means of tiny hooks. This led him to invent a synthetic fabric whose surfaces mimic the clasping properties of this natural seedpod.

Animals and plants have evolved solutions to the kinds of problems that often interest engineers and designers. Much current research in material science is concerned with actively examining the natural world, especially at the molecular level, for inspiration to develop materials with novel properties. This relatively new field of study is sometimes known as biomimetics, since it consciously attempts to mimic nature.

Researchers have investigated several interesting areas. For example, they have studied how the molecular structure of antler bone contributes to its amazing toughness, how the skin structure of a worm contributes to its ability to crawl, how the sea cucumber softens its skeleton and changes shape so that it can squeeze through tiny gaps in rocks, or what gives wood its high resistance to impact. These investigations have led to several breakthroughs in the development of composite materials with remarkable properties.

Predictions for future inventions that may be developed from these lines of research include so-called smart structures that design and repair themselves in a similar way to a variety of processes in the natural world. For example, engineers have envisaged bridges that would detect areas heavily stressed by vehicle movement or wind. The bridge structure would then automatically add or move material to the weak areas until the stress is reduced. The same principle might be used to repair damaged buildings. Other new materials that have been imagined are substances that would copy photosynthesis in green plants in order to create new energy sources. The potential impact of biomimetic research is so great that the twenty-first century may come to be known as the "Age of Materials."

 

Which of the following does the word “it” in paragraph 3 refer to?