阅读下面短文，根据短文内容选择最佳选项。
The Galapagos tortoise (加拉巴哥象龟) is a kind of tortoise. In Spanish, "Galapagos" is a word for "tortoises". The Galapagos tortoises gave their name to the Galapagos Islands because they were first found there.
Do you know what Galapagos tortoises look like? They can grow to be five feet long and weigh over one hundred and fifty kilograms. An adult tortoise is strong enough to lift a man.
Galapagos tortoises can walk for a long time without food. They can survive on very little water for a few weeks. The female tortoises lay eggs in a hole in the sand or among the rocks. The babies are very small with soft shells. The babies always face dangers from other animals. There are many animals which eat the eggs and the baby tortoises. In fact, only one in about three thousand survives to become an adult tortoise. Adults often live for more than one hundred years.
Now Galapagos tortoises are endangered because people eat the eggs they lay and the adult tortoises are hunted for their big shells. Also, the pollution of the Galapagos Islands is hurting the Galapagos tortoises as well. The number of them is becoming smaller and smaller. We should start to protect them.
中有机物的分子式排列规律，判断空格中有机物的同分异构体数目是（ ）
