Did College Board make a mistake here?

That the geographic center of North America lay in

the state of North Dakota was conceded by all

_______ establishing its precise coordinates proved

more divisive.

Which choice completes the text so that it conforms

to the conventions of Standard English?

A) involved:

B) involved,

C) involved

D) involved;

College Board is saying that the correct answer is D. Do you agree?

Explanation: "Choice D is the best answer. The convention being tested is the use of

punctuation within a sentence. This choice uses a semicolon in a conventional

way to join the first main clause (“That the...involved”) and the second main clause

(“establishing...divisive”). Further, the semicolon is the most appropriate choice

when joining two separate, parallel statements, such as here, where the

information following the semicolon contrasts with the information before."