There are three main differences between nuclear and mitochondria DNA (mtDNA): 1.) mtDNA is circular, whereas nuclear DNA is linear. (Both are double stranded) 2.) mtDNA is (generally) uni-parentally inherited, in most animals (not mussels though for example), mtDNA is passed from mother to offspring. from wiki
The correct answer is A. This is the case of using comma before a coordinating conjunction <em>and</em> because it links two independent clauses. We can observe that there two parts of this full sentence and each part is independent from each other. Both of them have their own subject and verb. Therefore, we are using a comma before <em>and</em>.
Mrs. Johnson's relationship with Mrs. Kennedy is polite and somewhat formal, while her relationship with Mrs. Connally is intimate and familiar. Mrs. Johnson has trouble finding the words to say to Mrs. Kennedy, but she feels free to cry with and console Mrs. Connelly.
