GWD-5-18

Q18:

Antarctica receives more solar radiation than does any other place on Earth, yet the temperatures are so cold and the ice cap is reflective, so that little polar ice melts during the summer; otherwise, the water levels of the oceans would rise 250 feet and engulf most of the world’s great cities.

1. is reflective, so that little polar ice melts during the summer; otherwise,
           
2. is so reflective that little of the polar ice melts during the summer; were it to do so,
           
3. so reflective that little polar ice melts during the summer, or else
           
4. reflective, so that little of the polar ice melts during the summer, or
           
5. reflects so that little of the polar ice melts during the summer; if it did

答案是B，其实我做的时候，看前半部分也觉得B是最佳答案，但是B答案后面的那句were it to do so,到底算什么结构呢？哪位能帮我解析一下吗？不胜感激！