4A. State and prove the 'Walras law' and mention (and prove) one of its important corollaries concerning the competitive equilibrium of an economy. 4B. If a competitive consumer chooses consumption program a = (15, 20) at competitive prices p = (2, 1) and consumption prgram y = (5, 7) at competitive prices p' = (3,5) then, under the usual assumptions, a) violates the weak axiom of revealed preference (WARP) b) does not violate WARP and exhibit direct revealed preference for r with respect to y c) does not violate WARP and exhibit direct revealed preference for y with respect to r d) does not violate WARP and does not exhibit any direct revealed preference between r and y e) NOPAC