单项选择题

There are two kinds of memory: short-term and long-term. Information in long-term memory can be recalled at a later time when it is needed. The information may be kept for days or weeks. In contrast, information in short-term memory is kept for only a few seconds, usually by repeating the information over and over. The following experiment shows how short-term memory has been studied.
Henning studied how students who are learning English as a second language remember vocabulary. The subjects in the experiment were 75 college students. 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 had the same meaning. Method, way, manner, and system would be four words with the same meaning. 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 sounds 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. The word "subject" in the passage means ______.