Which of the following are secondary lymphoid organs? (a) Bone marrow (b) Tonsils (c) Spleen (d) Thymus Choose the correct answer from the options given below:

1. A. (a) and (b) only
2. B. (b) and (c) only
3. C. (b) and (d) only
4. D. (a) and (d) only

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