The behavior of human beings can be very complicated. To find out how we learn, scientists often study the activities of animals. Sometimes they see how long it will take for a mouse to reach food in the center of a maze. Sometimes pigeons or other animals are taught to obtain food by pressing buttons. (A) In the first experiments in space flight, trained chimpanzees and dogs were sent into orbit. (B) They proved that living things could carry out certain activities while whirling about in space. (C) One early theory likened human memory to a muscle that had to be regularly exercised in order to function properly. This theory was eventually replaced by the notion that remembering was like writing, with experience the pen and the mind a blank page. (D) But eventually this theory was also rejected. In its place came another hypothesis--that human memory functioned like a complex and well-stocked library catalog. With access to a key word, you could look up any piece of stored information. But over time that theory has also been discarded. Human memory may, in fact, be too sophisticated and too complex to be explained through any one single simile or metaphor. When a paragraph contains two main ideas, it should be divided into two paragraphs. Choose the letter at the point where the passage should be divided. A) A B) B C) C D) D