English Grammar and Sentence Correction

Directions: The following sentences or parts thereof may have an error of grammar or syntax. Select the sentence(s)/part(s) thereof having the error.

(A) In this modern age there is a growing trend of viewing a country as a whole,
(B) with its people, topography, and general conditions in natural relation with each other.
(C) The process by which the nature control the imbalance created by human beings is peculiar.
(D) It is not easy to be sure today how accurate an observer he was.