语法填空（共10 小题；每小题1.5分,满分15分）

Americans love pets. In America, there are more families with pets    16   those with children. It is common    17    U.S. homes have some sort of pets, such as monkeys, snakes and even wolves. But Americans’ all-time favorites are cats and dogs, because dogs can offer protection from thieves and unwelcome visitors and cats can help get rid    18    the home unwanted pests. Beneath keeping pets, there lies a basic American belief: Pets have a right to be treated well. In Houston, Texas, dogs can have their dinner    19    (deliver) to their homes, just like pizza. Pets can even go with    20    owners on vacation. Besides, at least 75 animal welfare organizations exist in America    21    provide care and adoption services for    22    (home) and ill-treated animals. To pay for the high-tech health care, people can buy health insurance     23   their pets. Pets are as basic to American culture as hot dogs or apple pie. By now researchers    24    (discover) that interacting with animals lowers a person's blood pressure. Pets even encourage social relationships: They give their owners an appearance of friendliness, and they provide    25    good topic of conversation.

证明假设最可靠的方法是（）。

A．回顾性调查

B．现况调查

C．动物实验

D．抽样调查

E．实验流行病学