Naturalists have long known that the apes, our nearest relative in the animal 76. ______ kingdom, communicate with one and another through gestures, sounds and facial 77. ______ expressions. But it was always thought that only human beings can use words 78. ______ and sentences. In the 1960s, however, researchers set themselves the task of reach 79. ______ chimpanzees to communicate with humans. At first the scientists tried make them 80. ______ to speak. But no chimp ever managed to acquire a vocabulary of more than four words 81. ______ and even these were spoken with great difficult. The breakthrough came when two 82. ______ scientists from the University of Nevada decided to try America Sign Language, 83. ______ system of gestures used by deaf people. After four years, they had taught their first 84. ______ chimpanzees to use 132 signs correctly to communicate their wants and needs. 85. ______