Haliburton get theirs because with them it is 'one stop shoping.' They can provede trucks, drivers, cooks, buildings, security, mechanics, road building equipment, engineers, and medical people.
I also prefer to deal with contractors who can provide EVERYTHING. Sometimes price isn't everything.

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I love to relax by shooting cans.........AfriCANS, MexiCANS, Puerto RiCANS.................