【单选题】Stomach Ulcer Stomach ulcers are the cause of severe pain for many people. Doctors have been able to help lessen the pain of ulcers. They could not cure them. Now doctors have discovered a cause of ul...
【简答题】Experts do not really know when people began keeping fish as pets. But they say that people have been interested in fish for thousands of years. Some say the ancient Sumerians were the first civilizat...