The first antibiotic discovered by Alexander Fleming in 1928 was:

(A) Penicillin

Alexander Fleming's discovery of penicillin revolutionized medicine by introducing the first widely used antibiotic, which proved effective against a variety of bacterial infections.

(D) Jonas Salk

The polio vaccine was developed by Dr. Jonas Salk. He and his team conducted extensive research in the 1950s, leading to the development of the inactivated polio vaccine (IPV), which became widely used for the prevention of polio. The vaccine was a significant breakthrough in medical science and played a crucial role in the global effort to eradicate polio.

The substance that causes the clotting of blood is:

(A) Thrombin

Thrombin is an enzyme that plays a crucial role in the blood clotting process. It is responsible for converting fibrinogen, a soluble plasma protein, into insoluble fibrin threads. These fibrin threads form a mesh that helps in the clotting of blood, particularly at the site of injury or wound. Therefore, option (A) Thrombin is the correct answer.