Explanation:

An Rh-ve person, if exposed to Rh ve blood, will form specific antibodies against the Rh antigens. Therefore, Rh group should also be matched before transfusions. So, (c) is related.
During the delivery of the first child, there is a possibility of exposure of the maternal blood (if Rh
–ve) to small amounts of the Rh ve blood from the foetus. So, (a) is correct.
An injury or a trauma stimulates the platelets in the blood to release certain factors which activate the mechanism of coagulation. Calcium ions play a very important role in clotting