【单选题】

Your mind like your body is a thing where the powers are developed by effort. This is a principal use, as I see it, of hard work in studies. Unless you train your body you can’t be a good sportsman, and unless you train your mind you can’t be much of a scholar. The four mi- les a boatman covers at top speed is in itself nothing to the good, but the physical capacity to hold out over the distance is thought to be of some value. So a good part of what you learn by hard study may not be retained forever, and may not seem to be of much final value, but your mind is a better and more powerful instrument because you have learned it. "Knowledge is power," but still more the ability of acquiring and using knowledge is power. If you have a trained and powerful mind, you are bound to have stored it with something, its value is more in what it can do, what it can grasp and use, than in what it contains; and if it were possible, as it is not, to come out of college with a trained mind and nothing useful in it, you would still be ahead, and still, in a manner, educateD、

The title that best expresses the main idea of this passage is ______.A．Knowledge Is Power
B．How to Retain and Use Knowledge
C．Physical and MentalEfforts
D．The Trained Mind Is What One Needs