By Contract, Oil Shale Corporation Forbids Petro, Inc

By contract, Oil Shale Corporation forbids Petro, Inc., a wholesale buyer of Oil Shale's products, from purchasing the products of its competitors. This exclusive-dealing contract is allowed​

A) ​under any circumstances.
B) ​if its effect is to cause a competitor a loss of any business.
C) ​if its effect is to substantially lessen competition.
D) ​unless there is no effect on a competitor.