Choose the correct letter, A, B, or C. What causes coral bleaching?

Now listen carefully and answer Questions 31 to 40.
Questions 31-32
The Great Barrier Reef, stretching along the east coast of Australia, is the world’s largest collection of coral reefs, covering an area of around 300,000 square kilometres. The approximately 3,000 individual reefs that make up the Great Barrier Reef system are composed of over 400 different kinds of coral, the largest variety of corals found anywhere in the world. In addition, around 900 islands, ranging in size from tiny sandy cays to large continental islands covered with vegetation, are scattered throughout the area, particularly at the northern and southern ends.
Questions 33-38
Thousands of species of plants and animals live in the area. Reef habitats make up only about 7 percent of the region’s ecosystems. Other types of habitat in the area range from the shallow waters of coastal salt marshes to deep ocean habitat. Plant life in the reef habitat includes 500 different species of seaweed. The islands provide habitat for a wide variety of plant species. Those at the northern end support over 300 plant species, most of which tend to be woody, while the 200 species of plants growing on the islands at the southern end are largely herbaceous.
In addition to the 1,500 species of fish that live in and around the reefs, sea mammals also abound in the area. Among them are whales, certain species of which use the area as a breeding ground. Many types of reptiles can also be found living among and near the reefs, including crocodiles and several species of marine turtles. The former find their home in the saltwater marshes along the coastal areas, while the latter are attracted to sea grass beds. The land as well as the water teems with animal life. At least seven species of frogs, for example, inhabit the reef’s islands.
Questions 39-40
Unfortunately, this wondrous area of the world is threatened by climate change. Rising sea temperatures have led to an effect called coral bleaching, that is, large numbers of corals dying off, especially in the shallower areas of the reef. The Great Barrier Reef Marine Park Authority is attempting to find effective ways to deal with this issue that threatens the reef. One proposed solution involves shading the reef in certain areas to help keep the surrounding water temperatures down.