Sentence Correction

Prior to the 1800s, people dwelling in rural locations made up nearly 90 percent of the entire human population; in 2007, they decreased by 50 percent.

A. in 2007, they decreased by 50 percent.
B. in 2007, they decreased to less than 50 percent.
C. decreasing to less than 50 percent in 2007.
D. in 2007, this proportion decreased to less than 50 percent.
E. which decreased to less than 50 percent in 2007.

OA is D
Can an Expert explain what is wrong with B.