此题考查非谓语作原因状语.
（Because Australia has been seperated from other continents for millions of years,it has many...）
非谓语作状语,非谓语的主语就是句子的主语.本题Australia 就是前半部分的主语.
1.for many years 表明时态为现在完成时have seperated.
2.Australia 同seperate为被动关系.
结合1.2可知正确答案为
Having been seperated