Which are corrrect for choosing proppant A: for low permeability formations,the fracture mainly is requires to increases the length B: for low permeability formations,the fracture mainly is requires to increases fracture conductivity C: for medium or high permeability formations,the fracture mainly is requires to increases the length. D: for medium or high permeability formations,the fracture mainly is requires to increases fracture conductivity.

How many battle formations are there in Pit No. 2?

__________ is natural gas that is found trapped within shale formations. It has become an increasingly important source of natural gas in the United States since the start of this century. A: Methane B: Shale gas C: Coal gas

50. ___ A: They are newer than the formations in most caves. B: They were formed by sulfuric acid. C: They have been artificially preserved. D: They are smaller than those in any other cave.