One reason common names are not useful to biologists is that they
a. are in latin
b. can apply to more one animal
c. are too long
d. require the use of a dichotomous key

The correct answer among all the other choices is b. can apply to more one animal. One reason common names are not useful to biologists is that they can apply to more one animal. Thank you for posting your question. I hope this answer helped you. Let me know if you need more help. 

Iguess i would say diagrams would be most difficult subject too correct

The direction a chemical reaction occurs is represented by an arrow