题目

Sooner or later, most students are to take some form of English examinations. Often the more successful students are better prepared. However, sometimes students who do well have better test taking skills. These abilities really have nothing to do with understanding English better. They are skills that make taking the test easier, and therefore provide better results. Here are some very important—and often ignored—guidelines to taking a test successfully. Do not insist on completing each question before going to the next. This is extremely important. Remember one question may only 如图，在体积为1的三棱锥A-BCD侧棱AB、AC、AD上分别取点E、F、G，使AE：EB=AF：FC=AG：GD=2：1，记O为三平面BCG、CDE、DBF的交点，则三棱锥O-BCD的体积等于（　　） A、19B、18C、17D、14