If the SQL statement SELECT R1. A, R2. B FROM R1, R2 WHERE R1. K = R2. F AND R2. K = 10 is frequently executed, which indexes will prove most useful?

(a) Index on R1. K and index on R2. K
(b) Index on R1. A and index on R2. B
(c) Index on R1. K and index on R2. F
(d) Composite index on (R2. K, R2. F)