Answer: B) They are the sites at which microtubules attach to chromosomes. Explanation: They are the sites at which microtubules attach to chromosomes.

Answer: A) transcription Explanation: Protein synthesis at the ribosome is immediately preceeded by the transcription process.

Answer: C) Mitochondria Explanation: Cellular respiration is the multi-step process that creates energy from nutrients and food molecules. It begins in the cytoplasm of the cell, with the mitochondria functioning as the main organelle where the rest of the process continues and finishes. During respiration, glycolysis occurs in the cytoplasm of the cell.

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.