The second passenger ___ the bus was a middle-aged mam. A. getting on B

要填甚么首先要看懂句意“第二个上车的乘客是个中年男子”
很显然选项A中的getting on 是用于修饰The second passenger 的，

这句完整的应该是The second passenger who was getting on the bus was a middle-aged mam.
本句省略了who was

一楼说的对！