试题.在机场货物托运处,常用传送带运送行李和货物,如图1所示,靠在一起的两个质地相同,质量和大小均不同的包装箱随传送带一起上行,下列说法正确的是(A)匀速上行时b受3个力作用.(B)匀加速上行时b受4个力作用.(C)若上行过程传送带因故突然停止时,b受4个力作用. Questions at the airport cargo consignments, commonly used conveyor belt transport of goods and goods, as shown in Figure 1, two together rely on the same texture, quality and size of the box with the conveyor belt are up, the following statement is correct (A (B) When accelerating upwardly, b is affected by 4 forces. (B) If there is a sudden stop of the upstream process belt for some reason, b is affected by 4 forces.