For each question, choose the correct answer. Clever birds The wild birds known as ravens are thought to be very clever. In fact, they can solve some quite (21) ________ 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 (22) ________ it onto the ground, so that it would break and the bird could eat the nuts inside! Scientists also (23) ________ that the birds could actually use stones as tools to (24) ________ 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 (25) ________ at doing this. So for the next experiment, the birds were given a small (26) ________ 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! 21 A expert B complicated C heavy D confused 22 A letting B falling C dropping D leaving 23 A noticed B advised C watched D studied 24 A take B carry C make D check 25 A experienced B intelligent C correct D keen 26 A group B total C amount D number