Scientists have identified an asteroid, 2000 BF19, that is about half a mile wide and, if it strikes Earth, it can do tremendous damage to part of the planet but probably not cause planetwide destruction. - and, if it strikes Earth, it can do tremendous damage to part of the planet but
- and, if it would strike Earth, part of the planet could experience a tremendous amount of damage but it would
- and that, if it were to strike Earth, could do tremendous damage to part of the planet but would
- and that, if Earth is struck by it, can do part of the planet tremendous damage, but it would
- and that, if it strikes Earth, it could experience a tremendous amount of damage but
答案是C，我也觉得虚拟语气在这里会比较好，但我有些疑问：could do tremendous damage的主语是不是应该是that，而that指代it were to strike Earth，如果是这样，that 可以这样指代一个句子么？ | 
