As shown in the diagram above, a 2 -kilogram mass lies on a horizontal rough surface while a 1 -kilogram mass hangs vertically. The string between them is massless, and the pulley is both massless and frictionless. What minimum coefficient of friction will keep the blocks at rest?
(A) 0.1
(D) 0.4
(B) 0.2
(E) 0.5
(C) 0.3