Which answer is most appropriate?

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

Which is the best to express the meaning meant to be conveyed with the underlined portion? Thanks!