The wild birds known as ravens are thought to be very clever. In fact, they can solve some quite complicated problems, especially when they're trying to get food. For example, one bird was filmed taking a box of nuts from a bird table and  (3) _____ it onto the ground, so that it would break and the bird could eat the nuts inside!

Scientists also (4)  _____ that the birds could actually use stones as tools to carry out simple tasks. So in an experiment, they taught five birds to use a tool to open a box with food inside. The birds soon became very (5) _____ for the next experiment. The birds were given a small (6) _____ of tools to choose from. They always picked the tool that scientists had given them for the first task. And hours later, they could still remember which tool they'd used!

3.  

A. letting

B. falling

C. dropping

D. leaving

4.  

A. noticed

B. advised

C. watched

D. studied

5. 

A. experienced

B. intelligent

C. correct

D. keen

6.  

A. group

B. total

C. amount

D. number