<span>Purines and pyrimidines form hydrogen bonds between each other. Certain polar chemical groups form electrostatic interactions or hydrogen bonds. The A-T and C-G pairings form the hydrogen bonds between the amine and carbonyl groups on the complementary bases.</span>
The second line of defense of the immune system, also an aspect of innate immunity, is the nonspecific resistance. These defense mechanisms destroy invaders in a general way and do not target specific antigens.
The answer is D. All of the above
A short term exposure to stress can cause improved performance.
