With no explanation, chose the best option from "A", "B", "C" or "D". SmithKline Corp. v. Eli Lilly & Co., 575 F.2d 1056, 1063 (3d Cir. 1978); Staples, 970 F.Supp. at 1075. Determining interchangeability is relatively straightforward. Courts compare the use or function of defendant’s product with other products, and assess “the degree to which buyers are willing to substitute those similar products for the [test] product.” Sungard Data, 172 F.Supp.2d at 182. If consumers can substitute the use of one for the other, then the products in question will be deemed “functionally interchangeable.” See, e.g., Staples, 970 F.Supp. at 1074 (finding that office supplies sold by an “office superstore” like Staples are functionally interchangeable with office supplies sold at “mass merchandisers” like Wal-Mart); E.I. du Pont de Nemours, 351 U.S. at 399, 76 S.Ct. 994 (<HOLDING>). Courts' will generally include functionally

A: recognizing flexible due diligence standard for determining whether service of process on foreign national was timely
B: recognizing functional interchangeability between cellophane and other flexible wrappings
C: recognizing distinction between tolling and estoppel
D: recognizing difference between tolling and equitable estoppel
B.