Crows are notoriously poor and aggravating subject...

Câu hỏi: Crows are notoriously poor and aggravating subjects for field research. Keen observers and quick learners, they are astute about the intentions of other creatures, including researchers, and adept at avoiding them(A). Because they are so numerous, active, and monochromatic, it is difficult to distinguish one crow from another(B). Bands, radio transmitters, or other identifying devices can be attached to them, but this of course requires catching live crows, who are among the wariest and most untrappable of birds.(D) According to the second paragraph, crows are poor subjects for field research for all of the following reasons EXCEPT        

A They can successfully avoid observers.

B They are hard to distinguish from one another.

C They can be quite aggressive.

D They are difficult to catch.