Which sentence has an error?

a
It's not very hard to see why Harper Lee's latest novel has more pre-orders than any novel before now.
b
The jury could not make its decision because a fire alarm was pulled in the courthouse right when deliberation began.
c
The bear ambled slowly through the river until it caught a salmon, then it sat in the sun to eat it's lunch.
d
The team not only lost the game, but it also lost its ride back home since the bus had two flat tires.