Please answer the following question: Read the article and select the best answer. Article: A new study has found that it may be possible to train people to be more intelligent, increasing the brainpower they had at birth. Until now,it has been widely assumed that the kind of mental ability that allows us to solve new problems without having any relevant previous experience--what psychologists call fluid intelligence--is innate and cannot be taught(though people can raise their grades on tests of it by practicing). But in the new study,researchers describe a method for improving this skill,along with experiments to prove it works. The key, researchers found, was carefully structured training in working memory--the kind that allows memorization of a telephone number just long enough to dial it.This type of memory is closely related to fluid intelligence,so the researchers reasoned that improving it might lead to improvements in fluid intelligence. First they measured fluid intelligence of volunteers using standard tests.Then they trained each in a complicated memory task--the child's card game,in which they had to recall a card they saw and heard.During the course, they needed to ignore irrelevant items, monitor ongoing performance,manage two tasks at the same time and connect related items to one another in space and time. The four groups experienced a half-hour of training daily for 8, 12, 17 and 19 days, respectively.To make sure they were not just improving their test-taking skills,the researchers compared them with control groups that took the tests without the training. The results, published Monday in The Proceedings of the National Academy of Sciences,were striking.Improvement in the trained groups was a lot greater.Moreover,the longer they trained, the higher their scores were.All performers,from the weakest to the strongest,showed significant improvement. "Our results show you can increase your intelligence with proper training." said Dr Jaeggi, a co-author of the paper."No one knows how long the gains will last after training stops," he added, "and the experiment's design did not allow the... Question: .When the experiment was conducted, the researchers _ . Options: A: trained the four groups for the same period of time B: only made comparisons between the four groups C: compared the four groups with control groups D: trained the four groups together
A:
C