ACCorDing to thE sEConD pArAgrAph, Crows ArE poor suBjECts For FiElD rEsEArCh For All 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