题目内容：

阅读短文，根据短文内容，从短文后的四个选项中选出填入空白处的最佳选项.

A Smart Dog

Rico is a dog who lives in Germany, ____1.______ When his owner says the name of a certain toy, Rico can find it. In fact, he seems to know the names of 200 toys!

___2.___. They put his toys in a room and then told him to go in the room and find a certain toy. Since nobody was in the room with Rico, he had no help from anyone finding the right toy. The researchers did this test forty times. Rico found the right toy thirty-seven times!

Then the researcher tried something else. They put seven of Rico’s toys in the room and one new toy that Rico had never seen before. ______3.__ . This new toy had a name Rico had never heard before. Rico found the right toy seven out of ten times.!

Researchers cannot really say that Rico know words or languages. ___4.___They believed Rico can think about what he hears and thin about what he knows. In fact, Rico seems to thin and remember thins as well as three-year-old child.

From these tests, animal researchers know one thing for sure. Rico has given them a lot to think about.

A. Then they told Rico to go into the room and get the new toy.

B. However, these tests seem to show Rico is smart.

C. Some researchers in Germany wanted to test Rico.

D. His owner has trained him to find his toys.