U # 5 Discursive passage

2. The intelligence of birds has been suggested in stories, but   

3. Half of the birds in the experiment were immediately successfu; however,  

4. The birds promptly realized the advantage of using big stones, and so

5. The research showed rooks can use tools with ease, though

6. The rooks worked out the properties of different materials and as a result,
A     others needed several attempts.
B     experts think that they don't do this in their natural habitat.
C     they achieved their goal sooner.
D     confirmed a fictional account.
E     helped us to understand a mysterious event.
F     only scientific studies can prove this.
G     they were able to protect themselves.
H     consistently rejected one particular type.