The recurrence relation for the power series solution about x=0 of the differential equation y′′−y=0 is (for k=0,1,2,…) Select the correct answer. a) (k+1)kck+2​=ck​ b) (k+1)kck​=ck−2​ c) (k+2)(k+1)ck​=ck−2​ d) (k−2)(k−1)ck−2​=ck​ e) (k+2)(k+1)ck+2​=ck​