单项选择题

TEXT D
Psychologists study memory and learning with both animal and human subjects. The two experiments reviewed here show how short-term memory has been studied.
Hunter studied short-term memory in rats. He used a special apparatus which had a cage for the rat and three doors. There was a light in each door. First the rat was placed in the closed cage. Next one of the lights was turned on and then off. There was food for the rat only at this door. After the light was turned off the rat had to wait a short time before it was released from its cage. Then, if it went to the correct door, it was rewarded with the food that was there. Hunter did this experiment many times. He always turned on the lights in a random order. The rat had to wait different intervals before it was released from the cage. Hunter found that if the rat had to wait more than ten seconds, it could not remember the correct door. Hunter’s results show that rats have a short-term memory of about ten seconds.
Henning studied how students who are learning English as a second language remember vocabulary. The subjects in his experiment were 75 students at the University of California in Los Angeles. They represented all levels of ability in English: beginning, intermediate, advanced, and native-speaking students.
To begin, the subjects listened to a recording of a native speaker reading a paragraph in English. Following the recording, the subjects took a 15-question test to see which words they remembered. Each question had four choices. The subjects had to circle the word they had heard in the recording. Some of the questions had four choices that sound alike. For example, weather, whether, wither, and wetter are four words that sound alike. Some of the questions had four choices that have the same meaning. Method, way, manner, and system would be four words with the same meaning. Some of them had four unrelated choices. For instance, weather, method, love, result could be used as four unrelated words. Finally the subjects took a language proficiency test.
Henning found that students with a lower proficiency in English made more of their mistakes on words that sound alike; students with a higher proficiency made more of their mistakes on words that have the same meaning. Henning’s results suggest that beginning students hold the sound of words in their short-term memory, and advanced students hold the meaning of words in their short-term memory.
Hunter found that rats______.