The correct answer is letter A
Sinoatrial node (SA) or pacemaker or sino-atrial node: special region of the heart, which controls the heart rate. It is located near the junction between the right atrium and the superior vena cava and consists of a cluster of specialized muscle cells. The rhythmic frequency of these muscle fibers is approximately 72 contractions per minute, while the atrial muscle contracts about 60 times per minute and the ventricular muscle, about 20 times per minute. Due to the fact that the sinoatrial node has a faster rhythmic frequency in relation to the other parts of the heart, the impulses originating from it spread to the atria and ventricles, stimulating these areas so quickly, so that the rhythm of the SA node becomes the rhythm of the whole heart; that's why it's called a pacemaker.
Xenophobic attacks; causes and solutions. ... Xenophobia is the abnormal fear or envy or hatred of foreigners/strangers in a country by the citizens living in that country.As a result, Xenophobic Attack is just the attack of such foreigners by the citizens in that country due to the abnormal fear or envy or hatred of them ...hope this helps
Answer:
c.they are all parasites that harm their hosts by in or on it
