The best answer is B. The passage states each of the five well-know plant hormones ‘has more than one effect on the growth and development of plants”  and that, for this reason, “they are not very useful in artificially controlling the growth of crops”. Choice A is not correct because the passage describes some of the functions performed by the hormone auxin. Choice D is consistent with information presented in the passage, but by emphasizing the specific effect hormone have at the cellular level rather than the multiplicity of effects they have on the entire plant, D fails to prove the reason stated in the passage that the five hormones are not useful in controlling the growth of crops. Neither C is suggested by anything in the passage.