题目

如图4-5-4所示，长木板平放在地面上，用F=10 N的力拉动木板上的小木块使其匀速前进，但木板静止，则木板的受力个数为（    ）                                                                图4-5-4A.3              B.4                 C.5              D.6Even the best student in our class couldn't answer this question, so it ________ be very difficult. A. must B. could C. may D. can