Read the passage and fill in the gap with a suitable word. Coral reefs are the largest structures formed by living creatures (1) __________ earth; the Great Barrier Reef near Australia, for (2) __________, is nearly 2000 kilometers long. They are composed (3) __________ the remains of sea creatures, the skeletons of (4) _____________ become part of the reef when they die. Coral has (5)__________ used to make jewellery for a long time because of (6) __________beautiful colors, but now it has a new use. Scientists have discovered that it is suitable (7) ___________ replacing bones in the human body. Bone can be taken from another part of the body for this (8) ___________, but this causes the patient a (9) ___________ of distress because he must have two operations. Alternatively, metal can be used, but bone cannot naturally assimilate (10) ___________ a material. The use of coral is (11) ___________ completely free of problems, as it absorbs poisonous metals easily, which (12) ______________ harm the human body. On the other (13) ____________, its structure is very like that of bone and is less likely to (14) ______________ rejected by a patient’s body. Two researchers in France have already carried (15) _____________a number of successful replacements and, with luck, the success stories will continue.