高中英语.perhaps I ---- Swim across the nver,but in that situati

高中英语.perhaps I ---- Swim across the nver,but in that situation,I--- do that.
perhaps I ---- Swim across the nver,but in that situation,I--- do that. A,am able;couldn't. B,cowld;was able to. C,was quite able to;could. D,couldn't ;was able to. 请说明理由,谢谢.