The thymus ... A. is located in the abdominal cavity. B. are where T-cells reside and mature. C. all of the answers. D. increases in size in adults.

To answer the question about the thymus, let’s analyze each option:

1. Option A: "is located in the abdominal cavity."
   This statement is incorrect. The thymus is actually located in the upper chest, behind the sternum, not in the abdominal cavity.

2. Option B: "are where T-cells reside and mature."
   This statement is correct. The thymus is indeed the primary site for T-cell development and maturation, crucial for the immune system.

3. Option C: "all of the answers."
   Since Option A is incorrect, this statement cannot be true.

4. Option D: "increases in size in adults."
   This statement is also incorrect. The thymus is largest during childhood and gradually decreases in size and activity after puberty.

Final Answer

The correct option is B. are where T-cells reside and mature.