A series parallel combination of batteries consisting of a large number N=300 of identical cells, each with an internal resistance r=0.3Ω, is loaded with an external resistance R=10Ω. The number n of parallel groups consisting of an equal number of cells connected in series, at which the external resistance generates the highest thermal power is.
(a) 2
(b) 3
(c) 4
(d) 6
(a) 2
(b) 3
(c) 4
(d) 6