The radiator in an automobile is a cross-flow heat exchanger (UA_s = 10 kW/K) that uses air (c_p = 1.00 kJ/kg · K) to cool the engine coolant fluid (cp = 4.00 kJ/kg · K). The engine fan draws 30°C air through this radiator at a rate of 10 kg/s while the coolant pump circulates the engine coolant at a rate of 5 kg/s. The coolant enters this radiator at 80°C. Under these conditions, what is the number of transfer units (NTU) of this radiator?
(a) 1

(b) 2

(c) 3

(d) 4

(e) 5