A、His poor memory. B、His fatigue. C、His lack of diligence. D、His method. D女士说“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: [22] The biggest problem with me is the Korean vocabulary. It really makes me frustrated. [22] The pronunciation 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 1 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. [24] 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: [25] The last thing 1’11 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? [23] I have thought every word should be practiced as often as possible within a short time span.
W: No. [23] How the word is practiced is much more important than how often it is practiced.
M: You really do me a great favor, Teresa.