A 9 V battery is connected in series to four resistors as follows: R1 = 2 Ω, R2 = 4 Ω, R3 = 4 Ω, and R4 = 8 Ω. What is the total resistance of the circuit?

A. 0.5 Ω
B. 9 V
C. 18 A
D. 18 Ω