Mr. King lived with six hundred wild animals on the Greek Island of Kyklos. Ever since he left school, he had traveled all over the word collecting animals for his own zoo. He hoped to collect two examples of every kind of animal on his island. But he was afraid that people would find him someday. He wrote books about his travels and about his animals that he collected. The money from the books helped to pay for all the food that these animals ate. One day, when Mr. King was out looking for drinking water, he found oil. He needed money for his travels and for his zoo, and a little oil would buy enough water for what he needed all his life, but he knew that if he told anyone else about it, it would be the end of his zoo and his life's work. So he decided not to tell anyone else about what he had found, because oil and water couldn't mix. In the passage, "oil and water couldn't mix" means that "  _  ".
Mr. King needed not only water but also oil oil and water were rather difficult to be mixed together Mr. King's animals only needed water, not oil if he told the others he found oil, his life's work would be stopped