A、Happy moods. B、Good memory. C、Proper reasoning. D、Some training. D短文开头提到，心智技能会随着人变老而自然减弱，然而研究似乎表明，通过一些训练可以增强这些(心智)技能。故选D。

Some kinds of mental skills naturally decrease as people get older. [16]Yet research seems to show that some training can improve such skills. A recently published study also appears to demonstrate that the good effects of training can last for many years after that training has ended.
    Researchers at Johns Hopkins University in Maryland wanted to learn how long memory and thinking skills would last in older people who trained to keep them. The people were part of the ten-year research project They were taught methods meant to improve their memory, thinking and ability to perform everyday tasks.
    More than 2,800 volunteered for the study. Most studied when they were more than 70 years old. The volunteers took one of several short training classes meant to help them keep their mental abilities. One class trained participants in skills including how to remember word lists. Another group trained in reasoning. A third group received help with speed-of-processing—speed of receiving and understanding information. A fourth group—the control group did not get any training.
    Earlier results had established that the training helped the participants for up to five years. Now, lead study writer George Rebok says, the research showed most of the training remained effective a full ten years later. Professor Rebok and his team found that the people trained in reasoning and speed-of-processing did better on tests than the control group. They are wondering whether those effects which endured over time would still be there ten years following the training, and in fact, that’s exactly what they found.
16 What can improve mental skills according to the research?
17 What was the purpose of Johns Hopkins University’s research?
18 What does the term speed-of-processing refer to?
19 What do we learn about the effects of most training?