Answer:
C!
Step-by-step explanation:
The most common treatment for leukemia is chemotherapy, as stated in the passage, so that would be step 1. If chemotherapy isn't enough, a bone marrow donor is needed in order to introduce healthy leukocytes that can take over - meaning step 2 is finding a donor match. Step 3 would then be to introduce the healthy leukocytes from the donor into the patient's bloodstream, and step 4 has the leukocytes traveling from the bloodstream (step 3) to the bone marrow, where they can begin replicating.