I think option C is appropriate for you case.

But most of the time people expect 3 reasons so three paragraphs will be more desirable. The limitations and conditions can be briefly discussed in concluding paragraph too.

Thanks

        Could you help me ? How to organize my structure ?

        In yes/no answer essay, e.g. " Is animal testing nessary ? "

A. Can I put my body structure like this :    If I say yes,

        paragraph 1, I put my reason 1  

        paragraph 2, I put reason 2

B. Or can I put

        Para 1, I put reasons : reason1 and 2

        Para 2, I put condition or limitation of my reasons above : But animal testing must be strictly controlled and limited.

C. Or should I make other structure like : 

        paragraph 1, I put my reason 1  

        paragraph 2, I put reason 2

        paragraph 3, I put condition or limitation of my reasons above

Which one is the most suitable ? Or should I make other structure ? What ?