Answer:
Both Change A and B
Step-by-step explanation:
Given that chemical changes is a form of change of matters that deal with the combination of various substances to form a new substance entirely that has different chemical and physical characteristics from its parents' substances.
Hence, when Animal waste is acted upon by anaerobic bacteria to produce biogas it is considered a chemical change.
Similarly, when biogas burnt as fuel it is also considered as a chemical change.
Therefore, in this case, the correct answer is "Both Change A and B" are chemical changes.