1) In population genetics, the term "genetic drfit" refers to the process by which gene frequencies in a population change from one generation to the next as the result of random chance.

a) by which gene frequencies in a population change
b) by which gene frequencies in a population have been changed
c) that gene frequencies in a population change
d) of gene frequencies in a population have been changing
e) of the change of gene frequencies in a population

Correct answer is A, but I would like to understand correct usage of BY WHICH. Can anyone throw some light on appropriate usage and meaning of BY WHICH.

Why A is incorrect in the following SC:

2) The agreement, the first to formally require industrialized countries to cut emissions of gases linked to global warming, is a formal protocol by which 38 industrialized countries must reduce emissions of these gases by 2012 or face heavy penalties.

A) by which

B) for which

C) under which

D) such that

E) wherein

Source: [spoiler](1) Veritas; (2) GMATPREP; [/spoiler] OA-C