GWD 12-10

Q10:

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B,但我觉得C更好，为什么C不对呢？请指教

C就是错在漏is，or else也unidiomatc

little of ice和little ice没有本质错误

were it to do so, 很好的插入语！