Questions 17-20 are based on the following passage. You now have 20 seconds to read the questions 17-20. What is the purpose of the tank test

A．To find out how much air can be pumped into a plane.
B．To find out how much air passengers need to breathe at certain altitudes.
C．To find what would happen if the plane crashed in the water.
D．To find out if there are any weak parts in the plane that would burst under pressure.