谁知道这个知识点考的是啥

Devise a sampling plan for obtaining a representative BTU sample from an open top railcar of coal, where the contents are known to have changed and also stratified due to vibration and exposure during the 10 days of shipment.
A.Draw a slipstream sample by taking a representative slice of the bulk material as the car is being unloaded, being certain to capture all of the generated dust.
B.Receive a certificate of analysis from the supplier of a sample properly collected from the entire load at the time of loading, knowing that the coal got wet and dust blew off the open railcar.
C.Draw a quota sample with one sample off the top and from five more standardized depths in the railcar.
D.Draw a convenience sample from multiple points that can be easily accessed.