Questions 17-20 are based on the following passage. You now have 20 seconds to read the questions 17-20. Why is the tank test carried under water

A．Because the water pressure is greater than air pressure.
B．Because the first comets crashed in the water.
C．Because there is less damage when the explosion takes place under water.
D．Because it is the only way available for engineers to find out which part has cracked.