Practice Test — Study

Coral reefs cover less than one percent of the ocean floor, yet they are home to about a quarter of all marine species. Fish, turtles, sea stars and countless smaller creatures depend on reefs for food and shelter. Reefs also protect coastlines from strong waves and support fishing and tourism in many tropical countries. For coastal towns, healthy reefs can also draw visitors who come to swim, dive and watch wildlife, bringing income to hotels, guides and small restaurants.

Corals are tiny animals that live together in large colonies. They build hard skeletons of limestone, and over thousands of years these skeletons form the structure of a reef. Inside the coral tissue live tiny algae, which use sunlight to make food and share some of it with the coral. This close partnership gives healthy reefs their bright colours.

Today, reefs face serious threats. When sea water becomes too warm, corals push out the algae and turn white, a process known as coral bleaching. Without the algae, corals lose their main source of food and may die if the water stays warm for too long. Pollution from farms and cities, plastic waste and careless tourism also damage reefs. Rising sea levels and stronger storms add further pressure, because heavy rain can wash soil and waste from the land into the sea, covering corals in mud and blocking the sunlight they need.

There are, however, reasons for hope. In some places, local communities have created protected areas where fishing is limited and visitors must follow strict rules. Scientists are also growing young corals in underwater nurseries and planting them on damaged reefs. These projects are slow and expensive, but several have already shown new coral growth within a few years. Education plays a part too, as visitors who understand how fragile reefs are are more likely to follow the rules and avoid touching or standing on the coral.

Paragraph A Glass is one of the oldest materials made by humans. The earliest glass objects, mostly small beads, were made in Mesopotamia and Egypt more than three thousand years ago. Early glass was rare and valuable, and it was used mainly for jewellery and small containers for perfumes and oils.

Paragraph B Glass is made by heating a mixture of sand, soda ash and limestone to a very high temperature until it melts into a thick liquid. When this liquid cools, it becomes hard and transparent, but it does not form crystals like most solids. Because of this unusual structure, scientists sometimes describe glass as a material between a liquid and a solid in its atomic arrangement.

Paragraph C A major change came in Roman times, when glassblowing was invented. A worker could blow air through a long tube into hot, soft glass and shape it into bottles and cups. This method was much faster than earlier techniques, so glass objects became cheaper and far more common in everyday homes.

Paragraph D In the Middle Ages, glass became important in buildings. Cathedrals were built with large windows of coloured glass, made by adding metal powders to the mixture before heating. These windows told religious stories in pictures for people who could not read, and they filled the buildings with coloured light.

Paragraph E Modern glass is everywhere. Special strong glass is used in car windows and phone screens, heat-resistant glass is used in kitchens, and glass fibres carry internet data across oceans. Glass can be recycled again and again without losing quality, which makes it one of the most useful materials in a world trying to waste less. Builders also value glass because it lets natural light into rooms, which can reduce the need for electric lighting during the day. Researchers continue to develop new kinds of glass that clean themselves or change colour in bright sun.