Complete the sentences with the words. (Hoàn thành các câu với các từ)

can't have            could be                       may feel       

may hear              might discover            must have

1. Evidence shows that elephants …….. feelings. Experts say that they express sadness in a number of different ways.

2. Scientists think that when dolphins jump, they ………… happy, but they aren't sure.

3. If you listen carefully, you ………. the dolphins making noises to communicate.

4. People feel that microorganisms……….. emotions because they are too small.

5. It's possible that scientists …………. more about animal emotions in the future.

6. Some small creatures…………. unhappy at certain times, but we don't know for sure.