A、His poor memory. B、His fatigue. C、His lack of diligence. D、His method. D女十说“练习的方法比练习的次数更重要”，故选D。

W: Roger, any problems in your Korean study?
M: The biggest problem with me is the Korean vocabulary. It really makes me frustrated. The pro-nunciation and usage put me into great confusion as I keep forgetting them.
W: I guess first of all, you can narrow down the vocabulary sphere. I mean, try to learn "core" vocabulary, the words with high frequencies of occurrence.
M: That sounds quite reasonable. Do you think I can find in bookstores a book or dictionary where such kinds of words are listed?
W: Yes, definitely. Another thing, the first several occurrences of strange words are very important. Words are remembered best if they are learnt quickly with a few presentations. You may not have much trouble in remembering swear words, may you?
M: No. They’re very impressive and quite memorable. I don’t even need to practice them.
W: That’s it! For other words that don’t have such a strong effect, you have to make yourself highly attentive and sensitive. If not, later practice and repetition don’t seem to work so well.
M: I see.
W: The last thing I’ll say is that if you want to remember something for periods longer than a year or two, you need to space the presentations over quite long intervals of days, say 30 days. A more frequent interval doesn’t promise you longer remembrance.
M: Really? I have thought every word should be practiced as often as possible within a short time span.
W: No. How the word is practiced is much more important than how often it is practiced.
M: You really do me a great favor, Teresa.
12. What is the man’s biggest problem?
13. What is the cause of the man’s problem?
14. Why needn’t the man practice swear words?
15. What is an advisable way of longer remembrance of vocabulary?