Section 7 of the Clayton Act prohibits acquisitions, including mergers, where in any line of commerce or in any activity affecting commerce in any section of the country, the effect of such acquisition may be substantially to lessen competition, or to tend to create a monopoly. 15 U.S.C. 18; see United States v. Philadelphia Nat'l Bank, 374 U.S. 321, 355 (1963) (The statutory test is whether th