单项选择题A．nearest of all planets to the sunB．extremely hotC．little known to peopleD．biggest of all planets

单项选择题These references most probably appear ______.

单项选择题The terrible pain during a crisis is caused by______.

单项选择题According to the passage, which is NOT TRUE of sickle-cell anemia

单项选择题In the Middle Ages, the copying work of books was mainly done by______.