A、His poor memory. B、His fatigue. C、His lack of diligence. D、His method. DWhat is the cause of the mall’s problem?

W: Roger, any problems in your Korean study?
M: The biggest problem with me is the Korean vocabulary. It really makes me frustrated．The pronunciation and usage put me into great confusion as l 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 ale 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 lat 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 long 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.