what do you think of the picture?oh,it might be a girl -----

what do you think of the picture?oh,it might be a girl _____ a horse.A,to ride B,riding
谁能告诉我这个空应该选什么以及原因?
对不起，刚刚疏忽把题弄错了一点点
A；what do you think the picture is?
B；oh,it might be a girl _____ a horse.
A,to ride B,riding