Answer: C

Suppose the weights of the five sheep from light to heavy are a1, a2, a3, a4, and a5. The sum of all weights is 536 kilograms. Each sheep has been weighed 4 times, so the total weight of the 5 sheep should be 134 kilograms; because a1+a2=47, a4+a5=59, a3=134-47=28. The two heaviest sheep average 29.5 kilograms. If a5=30, a4=29; if a5=32, a4 cannot be less than a3, so the heaviest sheep cannot be 32 kilograms, but 30 kilograms. Therefore the answer is C.