Sentence Correction

National zoos involved in modernization have found the local citizens are difficult to reach, taciturn and are suspicious of innovations.

(A) the local citizens are difficult to reach, taciturn and are
(B) local citizens to be difficult to reach, taciturn and are
(C) that local citizens are difficult to reach, taciturn and
(D) local citizens are difficult to reach, taciturn and are they are
(E) that local citizens are difficult to reach, and taciturn and they are

OA C

Source : Gmatclub forums

Doubt : though the C is the only correct option, but it also lacks a Main punctuation. As per the rule to list three items you use X,Y,and Z. There should be a comma before AND. However C doesnot employs any punctuation before AND and still correct.

to me it is a Run on Sentence. Because "taciturn and suspicious of teenagers are considered one entity" and then the form turns out to be X,Y, which itself is incorrect.

Please explain.??