Fact 1: Mammals can also generate little bursts of heat by shivering. Fact 2: Polar bears are mammals. Given the two facts above, what can generate little bursts of heat by shivering?

i. alpacas.
ii. bacteria.
iii. animals.
iv. polar bears.
v. phytoplankton.
vi. rabbit.
vii. viruses.
viii. humans.
The answer is:
iv.