Select the most appropriate set of words from the given choices to fill in the blanks.

Every one will admit that swindling ones fellow beings is a necessary practice; upon it, is based really sound commercial success ___________.

A. sell what you cannot buy back

B. buy what you will sell to another at a higher price

C. buy cheap and sell dear

D. sell what you can, do not buy from a competitor

Please do not use chat terms. Example: avoid using "grt" instead of "great".