Carnivorous mammals can endure what would otherwise be lethal levels of body heat because they have a heat-exchange network which kept the brain from getting too hot.
7. Carnivorous mammals can endure what would otherwise be lethal levels of body heat because they have a heat-exchange network, which kept the brain from getting too hot.
A. which kept
B. that keeps
C. which has kept
D. that has been keeping
E. having kept
B Correct. (that) introduces a restrictive clause; (keeps) indicates a current situation and is consistent with the other verbs in the sentence.