What is a pressure gradient?
O A. An increase in pressure toward the west
O B. A region of extremely high pressure
O C. A region of stable pressure
O D. A difference in pressure

Consider a production line with five stations. station 1 can produce a unit in 9 minutes. station 2 can produce a unit in 10 minutes. station 3 has two identical machines, each of which can process a unit in 12 minutes (each unit only needs to be processed on one of the two machines. station 4 can produce a unit in 5 minutes. station 5 can produce a unit in 8 minutes. which station is the bottleneck station?