参考答案

正确答案：A
解析：Which sentence is the most proper one to describe the author's attitude?
四个选项的都是描述作者的形容词，由此可以推测，本题考查内容可能是关于作者的态度。文中最后一句提到" there are so many billions of planets that this would still leave a vast number on which life could exist".．还有无数的星球为生命的存在提供可能。所以作者的态度仍然是乐观的，故正确答案为A)。