In most major metropolitan areas, the number of hate crimes per capita is significantly lower in neighborhoods in which the population is racially mixed, that is, no more than one third of the area's inhabitants belong to the same ethnic group; it is theorized that a person is less likely to commit a hate crime against someone who he regards as a neighbor.

A) that is, no more than one third of the area's inhabitants belong to the same ethnic group; it is theorized that a person is less likely to commit a hate crime against someone who he regards as a neighbor

B) that is, no more than one third of the area's inhabitants belong to the same ethnic group; it is theorized that a person is less likely to commit a hate crime against someone whom he regards as a neighbor

C) that is, no more than one third of the area's inhabitants belongs to the same ethnic group; it is theorized that a person is less likely to commit a hate crime against someone whom he regards as a neighbor

D) that is, no more than one third of the area's inhabitants belongs to the same ethnic group; it is theorized that a person is less likely to commit a hate crime against someone who he regards as a neighbor

E) that is, no more than one third of the area's inhabitants belongs to the same ethnic group; it is theorized that a person is less likely to commit a hate crime against someone whom he regards to be a neighbor

The OA is C.

Wow. What is the difference between C and D? What difference makes the word "whom"?