I believe all eukaryotic cells have DNA
It is B. It can not be A because the salinity of the ocean is not low it is high. It can not be C because the temperature does not effect salinity. It can not be D because it would not increase because a river does not have any salt in it.
An organism can "make a living" and survive in its niche.
DNA strand and its complementary strand are held together by hydrogen bonds. One strand will be bonded to a T (thymine) on the opposite strand by 2 hydrogen bonds.