题目

   68                           It can make you confident when you are studying. But unluckily, most students don’t know how to take notes.     69                                           If your teacher writes notes on the blackboard, that’s great. You can copy them or write down the most important facts of all in class. Different teachers do things differently. For example, some teacher may focus on lots of dates and facts in class but they only write the important ones on the blackboard. Other teachers may F1和F2是共点力，根据平行四边形定则求合力F，作图正确的是（　　） 　 A． B． C． D．