Does a defendant have to prove that he or his co-conspirator had knowledge or foreseeability of drug dealing?

"[W]hen the calculation of the base offense level depends on the quantity of narcotics attributable to a defendant who was a member of a narcotics distribution conspiracy, we have concluded that all transactions engaged in by him or his coconspirator may be considered if the transactions were either known to him or reasonably foreseeable to him." United States v. Negron, 967 F.2d 68, 72 (2d Cir.1992). The defendant has the burden of proving lack of knowledge and lack of foreseeability. Id.